You're very optimistic, OP... but I hope this works out, nonetheless.
Why did you pick it?
I logically weighed up the options available to me versus my ethics. The first thing that came to mind was that no one else needed to die in this story; to force death on anyone else would be no different than what was believed to be so evil about the reapers. To me, ending life when you have the choice not to makes you far, far worse than the reapers.
Each reaper represents a nation built up of harvested species. The geth don't deserve to die, nor does EDI. I wouldn't make that call. That's genocide. That's like asking me if I could infect all life on earth with a deadly plague 'for the greater good.' I couldn't. I wouldn't. There would have to be another way. I wouldn't become that monster. I refuse to become that monster.
So that left me with Control and Synthesis.
Control is a very interesting option, but I worry that it would result in quasi-fascism. That bothers me. I could see Shepard going insane with power. Yes, this is headcanon on my part, but it is entirely possible, since he's only human. He might not, but having that hanging over my head as something that might happen... well, that just brings us right back to square one.
So, Synthesis, then. It all comes down to deductive reasoning. Destroy would turn me into a genocidal monster, I wouldn't be able to live with myself. Control might result in further craziness. So I needed to look at Synthesis to see what it brought to the table, to see if there was an option for me, to see if there was an option which would be compatible with my sense of ethics.
So what does Synthesis do? The first thing it seems to do is bring synthetic capabilities to organics. This means that organics can now upgrade themselves, but it also means that organics have access to a consensus where thoughts and feelings could be shared. That could lead to fun things like being able to backup one's brain, or being able to leave one's body behind all together. That raises fun questions like: What would it be like to be a spaceship? EDI and the geth know the answer to this, but we don't, so as organics we're jealous of them.
The Catalyst said that Synthesis was, in part, there to mitigate that jealousy by providing to organics what synthetics took for granted, so you can see that as the end result. Are you ill? Backup your brain, clone a new body, and problem solved! Do you want a different body? Build one, dump your brain into it, done! Do you want to teleport halfway across the galaxy? Upload your brain, convert the old body to resources, clone on arrival, dump the mind into the clone and done!
This would solve so many problems. This would allow humanity to transcend the human condition. We would no longer be prey to things like sickness or the limitations of our own bodies, we would become something else, something new, something better. As a transhumanist, I believe the purpose of humanity is to try to strive to create a better tomorrow, one where we can leave nature behind and forge our own path. Synthesis would allow us to do this, and it would take away a lot of today's suffering.
Synthesis would help all those people who're paralysed, or in comas, or deathly ill.
On the synthetic side, it would upgrade them so that they could understand our feelings and perceptions, it would essentially allow them to have and process empathy and sympathy, so that they could relate to us. So the organic thoughts entering the consensus wouldn't be seen as a disorganised cancer by the synthetics, they'd be able to perceive why we are as we are and accept us for that.
Most importantly, though? Synthesis is optional. That's canonical, the game mentions it.
So, at the end of it, I just had to ask whether Synthesis was compatible with my sense of ethics. I get to end suffering, upgrade the human condition, create a lasting empathy between all species (organic and synthetic), and more. So my answer was hell yes it is, it's very compatible.
As a person of ethical inclinations, Synthesis was the only choice I could make in good conscience. I, and my characters, are not mindless butchers or genocidal madmen. Synthesis was the only ending I could choose.
How did you see it working out past the slides?
I've pretty much explained this above, but yeah. I see galactic empathy, I see people being able to relate to others more. If two people are having a problem, they can share their thoughts and feelings in the purest of forms. It would change everything, a part of how it's possible to be so unconscionably cruel is that you don't have to feel the suffering of others. Synthesis would introduce people to that, they'd be able to feel how others do in regards to their actions.
Plus, I'd get to be part of a consensus as a living spaceship. And that's bloody awesome. I mean, could you imagine how it would be to be a living organism in space? Not in a ship, but as a ship. And there are all those other boons I mentioned, such as putting an end to illness, allowing people to choose the bodies they want, instant teleportation and so on.
I see a better council, and I see it taking cues from both the organics and synthetics. I see a new kind of politics being borne which is similar to that of the geth, where every sapient entity gets to vote on every agenda. And everyone can share the thoughts and feelings of those who're campaigning for any kind of change. It would eradicate xenophobia, by and large, and it would bring the galaxy together with a greater sense of oneness and understanding.
It's the Singularity, really. So I suspect that currency as we know it will be done away with, and currency based upon one's effort value and imagination will be instilled instead. Basically, ideas and the execution thereof will have effort values attached, and those will become the new currency. And that'll be a transitional system to even better and more egalitarian systems.
It's not all roses, though. Those who opted out of Synthesis will become ever more militant, and I'm certain that the reapers will show their faces again.
What happened within the game that influenced your choice?
My envy of the geth was a large part of it. When the Catalyst spoke of synthetic envy, I could completely understand that position. Plus, there was a lot of suffering presented within the game that could be solved by something like Synthesis. One of the most important elements though was that I felt that too many people had died already in this macabre story, and I wanted to create a scenario where no one else would have to die. I can be an idealist when I want to be.
Seeing how much effort it took to get the quarians and the geth to understand each other... well, that was the nail in the coffin, really. It proved that something like Synthesis was needed.
Why didn't you go for the other endings?
Ethics. I couldn't, in good conscience, choose endings that would result in so much death. Destroy would paint me as a genocidal, uncaring beast. A monster. Not even human at all. And Control just left me iffy as to whether a human could handle that much power. My sense of ethics dictated my choice.