I have been scrolling through this thread, and others, taking in both sides of the argument, and reminding myself of past events, and while originally upset with the ending, I've changed my mind. Don't get me wrong, I'm still heartbroken. I'm mourning the end of Shepard’s story, the characters that I love, and the Mass Effect universe as we knew it. I didn't realise, til the end, just how much I was emotionally invested into this series. I've never experienced anything like it. And judging by the strong reaction here, everyone feels the same. But I'm now trying to think logically. I would have loved the definite closure. Seeing Shepard reunite with her crew, her LI, just to know exactly where my Shepard ended up. So I'm still sad at the ending, but I think I understand it now. It's a wall of text, I know, but I just need to air this out... for myself if nothing else!
Looking back it seems inevitable that there would only be three choices. Destroying them by conventional means was never a possibility. This was made clear repeatedly througouth the series. There simply wasn't enough firepower to match them. The only solution was the catalyst. To destroy the Reapers via the catalyst was to destroy their technology. I hadn’t considered this at all. But this tech (the Citadel and relays) were there to help them repeat the cycle of destruction. This was their function. Organic life has never developed along a natural path on its own. They were guided. Their futures decided. So the destruction of the Citadel and the mass relays, in hindsight, seems obvious now. Destroy the reapers, along with all traces of their existence.
The past two games were hinting at our choices. Saren believed our salvation was synthesis. Illusive Man believed it was in controlling them. But the only true solution was in destroying them. The catalyst told Shepard that, being part synthetic, this choice would kill her too. But it was a lie. It wanted Shepard to choose the other two options because then they would win. But Shepard
does survive. Shepard saves her friends, the person that she loves, Earth and all survivors of the war. That's a victory. The reapers have lost.
The catalyst couldn't have destroyed the systems when it wiped out the relays, because Earth was saved. Everyone there is fine (evident by the scene at the end of the credits), and life continued. So while the military of various races would be stuck in the Sol System, it stands to reason that, like Earth, the other planets of other races still had survivors of the reaper attacks also (Wrex for example mentions that Eve is safe on Tuchunka, and is pregnant!). They also know of Shepard’s story, and they too would eventually rebuild and flourish as well.
Another thing to consider. As i was, a lot of people are wondering where the hell the Normandy ends up. My thoughts now: It doesn’t make sense that the Normandy is in another system in some random jungle. The Normandy was in the battle above Earth. It must have simply been outrunning the wave. With the relays destroyed, how could they be anywhere else? The link to other systems is gone. So, I like to think that it crashed landed on Earth somewhere untouched by reapers. It’s a remote location. With no populace there, what reason would the reapers have had to destroy that? They are there to harvest people. Not destroy planets. That parting image was one of hope.
All of Shepard’s exploits weren't for nothing. Shepard left her mark throughout the entire galaxy. Everyone knows Shepard’s name. Shepard united an entire galaxy to save it. A huge feat in of itself, but Shepard also helped in so many other ways. Shepard helped the quarians regain their home world; helped cure the Genophage for the krogans. Shepard left peace and hope for everyone. But it was not without heavy cost. Losing the relays is devastating. The non-human races are cut-off from their home worlds. But with the Citadel in the Sol System, even if they hadn’t pledged help to Earth prior, for the sake of the galaxy, it was where they needed to be in the end regardless. There was always going to a heavy price for victory. I’m sure they would be comforted at least knowing that their home worlds and people are finally safe. Now all organic life can grow on its own. Any one that survived would know what they have lost, and the mistakes they have made, but they would also have the knowledge to begin rebuilding. Learning space travel all over again with their own technology. Over time, I'm sure it will be as it was. But this time free of the reapers.
As for Shepard. Shepard woke up in a chunk of Citadel debris that would have I imagine been floating above Earth somewhere. I like to think that she found a way to get out/alert anyone that could help her/him. Everyone, including the crew, would eventually have heard of Shepard’s miraculous survival, and Shepard could finally have her happy ending.
Anyhoo, maybe I've missed things, but it’s just my interpretation and its made me feel better