I'm going to go wildly against the grain and say ME3's ending was not a train wreck. Far from it. What it did was take a theme that most completely overlooked or ignored (Organic vs Synthetic) which had been present in every single game since ME1, and made in the motivation of the antagonists. Unfortunately since people mostly overlooked or ignored it as a theme, that conflict being the primary driving force of the antagonist made little sense to most.
As a writer, who has spent a lot of time writing, Drew makes a lot of good points. You can't always know how a story will end. Or, sometimes you do, but as you go along, the story goes off in it's own direction and makes the ending you originally wanted irrelevant or impossible to achieve. And like people have said, trying to railroad your story to fit a set ending when the story will not go in that direction is bad.
But honestly, the process isn't perfect, and it's not a science. I mean, if anyone owns the ME1 artbook it clearly indicates that the Geth were originally going to be anthropomorphic bats at first. And look at what the Geth are now? Writing is a journey in which even the destination isn't always certain or clear. Even when you know where you think you want to go, you may wind up going in a completely different direction.
It's like going on a road-trip where your own real goal is to travel X number of miles, no matter where the trip ends.
Definitely agreed. Ironically, all three aspects of the endings are present throughout the trilogy but they are sidelined in the face of some major threat. Bringing them forward in the last minute of a trilogy as being the main reason for the Reaper existence rightfully feels disconnected from the rest of the trilogy.
Instances of synthetic/organic theme in
ME1:
Presidium AI, Rebekah and Michael (Synthesis), Rogue VI on Luna, Lost Freighter (machines keep a brain-dead person alive), geth
ME2:
Wrecked Merchant Freighter storyline, Overlord DLC (Control), Shepard's resurrection (Synthesis), Legion and his LM, Tali's loyalty mission, EDI, proto-Reaper (Synthesis)
ME3:
EDI arc, geth/quarian conflict, the Reapers themselves
As you can see, the theme does not come out of nowhere. But, on the other hand, it does not have as major presence as, say, in Deus Ex: HR where the entire game is built on the idea of augmentation and its positive and negative sides. That's why when you are faced with a similar choice in Deus Ex: HR ending it doesn't feel disconnected. Mass Effect introduced and used the organic vs synthetic theme, but due to the lack of planning or something else, it wasn't developed deeply enough to feel like a main theme of the trilogy.