You are aware, of course, that this is already an interpretation of the endings, right?
Having said that, I've played with the idea that the original endings were as undefined as they were because they were meant to replicate this uncertainty of outcomes you mention, that if you die - and Shepard does die - you'll never know the outcome of your decision. I don't know if such a scenario could work in a game, but I may have found it acceptable if the story didn't hint that the outcome was bad - namely, that dark age of the galaxy. I set out to save my civilization, and had I left the story with an indication, if not the certainty, that the outcome would be good, that might've worked. Instead, I left the story not with complete uncertainty, but with the suspicion that the writers meant for everything I set out to save to be destroyed.
I'll also say that for this to work, it would've been necessary to have the choices explained to us by someone other than the antagonist. How can you expect anything but bad outcomes if all the options are the antagonist's?
As for the basic position, maybe I'm unusual in that, but I often think about how what we do now might affect the world in 10000 years, especially with regard to global warming. I'm afraid Earth will end up like Venus, and that I won't live to see the time doesn't make that much of a difference. The same with transhumanism. It's not an idea I can reasonably expect to profit from, but I'd be happier if I could expect it to take root some time in the future.
I'm not entirely sure what you mean by the first line. Am I aware that I'm not the first person to read the ME3 ending through an existentialist lens? Well...yeah of course I'm aware of that? But like...it's still cool to chat about it? Or are you asking whether I'm aware that what I'm saying is only an interpretation? Again, yeah, but...see above? 
As to the rest of your post, I'd be happy to explain why I, at least, felt that the endings did work, perhaps for the very reasons you didn't. As a foreword to all this, I'm not expecting you to change your mind, and that's fine. But you genuinely seem interested in discussing this, which is cool. So!
Your position, as I understand it, is that there was too much uncertainty, or hinted negativity, for the ending to work in an existentialist fashion.
Firstly I suspect I reacted more favourably as I didn't interpret the endings in such a depressing fashion. All three endings stop the Harvest. It is true, none of the three endings allows your culture to remain completely unchanged, but one of my primary concerns was how they would resolve the Reaper threat without trivialising it. The scale Mass Effect 3 asked me to buy into, I feel, demanded a fundamental paradigmatic shift in its resolution. Destroy is the most bleak, I agree. But I think it's fitting that at least one of them is. And I find the possibilities of synthesis both terrifying and joyful.
I know another common feeling is that the destruction of the relay network is very dark in its implications, but I thought three endings lined up pretty well on that front, when you consider that the relays were a malignant vine trellis created by the Reapers. You have the politically nihilist reaction of destroy - "all symbols of our oppressors will be destroyed no matter the collateral damage" - you have the existentialist reaction of control - "things mean what we choose for them to mean, and we choose for the relays to be ours now, not theirs" - or the transhumanist response of synthesis - "the relays are no longer required, we will invent something better".
The cycle ends with death (Shepard's) and begins again with uncertainty, but also hope. That's what the Eden imagery after the crash says to me. We survived. Your culture may not have survived completely intact (and I agree, the implications for its future vary wildly from ending to ending), but I don't feel, overall, the tone is bleak and terrible.
But that's me. Even if I did feel the same way you did, I'm not sure I'd agree that the uncertainty itself is the problem because the uncertainty is inherent in the idea of an existentialist ending. Like...that's fundamental to it: how do you deal with a lack of external meaning. There is no perilous freedom if you're pretty sure you know what will happen.
I'd go further and say that what they did could only have been done via a game - via an interactive medium. They spend so long establishing the rules of the universe and giving you feedback in various ways, and then you literally experience that being taken away from you at the most important moment. The final, critical decision - what is the result of that? You'll never definitively know. What did it mean? Nothing except what you choose for it to mean.
I'd argue it worked too well. Existentialism isn't a mainstream philosophy because a lot of people find its underlying concepts depressing. People wanted...not necessarily a happy ending but a certain one. Instead they got what they got, and they experienced it in a much more immersive way than they would have in a book or on film.
I mean, clearly it didn't work for a lot of people, so perhaps you're right - perhaps it isn't something you should attempt in this medium. But I also can't help but feel that going for something different would mean veering away from an ending where the experience of playing the game is integral to the experience of the narrative - this real symbiosis of form and function, and I can't help but feel that would be a shame.
Oh - I haven't covered the Catalyst as an antagonist. Here I suspect we just disagree. I liked finding out what was actually going on (I mean, I played it first before Leviathan came out). I liked that shift and reveal. Then again, I also loved the technological themes in the games well before ME3 and was thrilled they were coming into play in full force in the ending. I didn't feel that the Catalyst was necessarily any more inherently untrustworthy than TIM at the start of ME2.
I can read the Catalyst as a brutal response to a valid concern, or as an ironic example of unnecessarily creating the very thing that you feared. I don't feel the game requires you to agree with it. Again, like TIM, you may be acting for very different reasons, even if your goals, briefly, align.
The choices you have are imperfect and you don't have enough time to decide. You may not trust your source of information. But you do have to decide anyway. The whole set up feels dreamlike to me in a way that works for me and feels intentional. It adds to the instability.
Me too. I liked the esoteric nature of the endings and being able to interpret them in different ways... even to the point of being able to interpret the point of Shep's "ending" at vastly different points in the game. I can even view the Catalyst as either an antagonist or as a "construct" of Shep's own mind and him/her debating with "both sides" of himself/herself merely about the value/effect of the life he/she had led. In our own dying moments, it's really all any of us can do. We hold different beliefs about what happens "after" death, but all any of us really knows for sure is that we do die whether or not our "work" here on earth is finished.
In part, I think this desire to continually re-interpret the endings in different ways is what keeps bringing me back to the game... not out of sense of dissatisfaction, but of curiosity about how constructing Shep throughout the game to be a different person who believes in different things affects ultimately how I feel about the "choice" he/she might desire to have made... had he/she lived to actually make that choice.
Personally, I also tend to believe that the actual "result" of the Crucible is never revealed (nor was it ever intended to be revealed) in the game. Throughout the game, the idea is repeatedly expressed that no species in the entire galaxy actually knows what it does or who originally created the plans for it. They believe it to be a "weapon of some sort" and originally believe the Protheans created it but then learn that the concept began many cycles before that. The final scenes do imply that Hackett determined that it wasn't firing by having attempted to fire it himself remotely. I have to wonder if it perhaps actually fired "something" without Hackett realizing it and that no one in the galaxy anticipated (and without Shepard ever having to try to activate from the Citadel).
So, I choose Shep's ending course of action based on what he had done in the game. For example, if he/she had rewritten the heretics and sided with Javik about how to go about controlling "inferior" races and bought into Cerberus' ideals, I pick the control ending. If he/she united the geth and the Quarians and encouraged EDI to become more and more self-aware, I pick the synthesis ending. If he/she remained focused on the Alliance's desire to destroy the reapers and was about being a "soldier and not a politician," I'll pick the destroy ending. I also play around with how leaving out bits of conversations and entire missions affect how "I feel" what Shep would base his/her choices on. The game, overall, really has little to do with whatever I personally believe in IRL.
Yay! Someone else who likes it!
I don't particularly view the Catalyst conversation as a literal dream, but I definitely agree it has dreamlike qualities, and in a literary sense is trying very strongly to evoke that sense of having ascended to a place above the world, before having to leave it. It's definitely a death-like transition. And I can see why interpreting it along those lines in a more literal way would be appealing.
I definitely have personal opinions on what the best choice is, but I also agree, the end decision should ultimately reflect the Shepard who makes it. My first Shepard represented my own opinions more strongly and ended up choosing synthesis. But I was surprised and really pleased that as I played through it with other Shepards, other choices became viable, or even made the most sense. In the end, I played through with three Shepards and each chose a different ending. Even though I hate destroy on a personal level, it was the right choice for one of the characters. A truly pyrrhic victory, in my opinion, but what he would have done.
Answer: No. Your chocies exist because Bioware allows it and will end because Bioware demands it
Answer: No. We'll paint smiles on everyone so you feel better though
It sucks
Thank you for that stunning contribution to the discussion.
I totally agree.
The concept of choice in the game makes me think about the oral tradition of epic stories. In the Middle age, stories were told and told again but it wasn't the same exact story (it coudln't be the same exact story). Some insisted on one aspect, some on another. If you take Merlin, some insisted on his human aspect, some insisted on his "demon" origin. The same character could be interpreted differently while it was basically the same story. Shepard can be paragon or renegad, the story is the same (Shepard tries to stop the reapers). The stargazer scene creates that relation with the oral tradition of stories.
At the same time, the notion of choice is also very close to the philosophical concept. I mean when there is choices, there is no freedom. During the whole game the choices can be seen as determinism and that aspect is justified by the stargazer scene (the kid asked if it really happened and he wants another story as if the whole trilogy was just a story - so the stargazer scene creates a meta level). So if the whole trilogy is about choices that are determinism because of the narration, and ingame because of the cycles, the only choice that can lead to freedom is the last one. We can see the last choice breaking the determinism of the narration, creating freedom from the cycles and from the narration (so the player is free to imagine the event post-choice). We are trapped in the cycles and in the narration, the purpose of the game is to break it. That's also a possibility to see it.
Yes, fantastic point on the issue of oral traditions. Part of what fascinates me about the ending is the way that all three endings are functionally identical (you go up to a platform above the sky and die choosing how you will end, and restart the world) but symbolically diverse. How you stop them, why you stop them, how you felt, that's what matters, that's what makes the story unique. All the nexus points of choice replicate the fragmentation of history via oral tradition. Shepard fought with the Krogan, but did he trick them, or save them? Shepard was there when Mordin died, but was he responsible or a witness? The first human spectres were lovers - no, no wait, that's not true, Shepard loved the Asari who found the Crucible and together they saved the galaxy! The stargazer could be telling any one of our stories.
All the writing is designed around herding us back to these crucial moments - these story points that can't be avoided. As you say, the only way to escape that determinism is to reach the end of the story, to reach the end of the writing. But that also means stepping beyond the point where we can know anything. We know what we meant to do, and in letting us know what we were attempting, I feel the game discharged its narrative responsibility. It explained the cage - the cycle of the Reapers - it explained what we were trying to break. And then it let us break it. The scope of the game is unimaginable - like biologically, our brains can't conceptualise the the true scale of the Reapers. But I feel like the metatextual subtext helps here by adding another dimension and letting it feel more all-encompassing.