I HOPE someone reads this as it should cover everything I've been saying in one place. Without being as confrontational as I usually am (intentionally or not), I am actually curious what Shamus thinks when it's laid out like this:
They don't need to make everyone happy, just not piss everyone off. This is a point a lot of people talking about the subject and the problems with the ending being changed seem to miss. Most endings of big franchises do not have everyone entirely satisfied, and a lot of people saying they would have done things differantly, but the majority at least tolerate it, with factions of lovers and haters. Stories that end with an climax that almost EVERYONE loves become huge phenomenas because of it, those that almost everyone hates... well we're looking at that now.
People also overlook the comments made by Devs about the game and how the ending was going to work, with clear statements being made that this was the kind of ending we were not going to get, and that we were indeed going to get the answers. If you've done any reading on their planned "ending app" and discussions about it, you'll find that they guys doing the ending didn't have it planned out until the last minute, and also decided specifically NOT to do what the devs had been promising through the rest of the game's development cycle, turning this into a giant intent to defraud the customer base through false advertising.
What's more we know as far back as ME1 that Bioware claimed that they had the entire trilogy plotted out from the beginning. The very fact that they were working on an ending seems to show they were made to change it. Probably for financial reasons related to wanting to turn it into a massive franchise and make that as easy as possible.
Perhaps most damning of course is how the ending is monetized in ways that didn't really exist as viable channels back when ME1 was coming out. The climax of the game is influanced by the resources you collect, but there aren't enough resources in the game to get a perfect ending without raising your readiness rating which is done through multi-player that involves a "pay to win" (or pay to save yourself a lot of time and frustration) mechanic. Not to mention the declaration that they planned to include other products to raise readiness like some kind of connected App game called "Mass Effect Infiltrator" (I think that was the name) which was another product for people to buy towards that end.
Circumstantial evidence or not, it paints a very damning picture of Bioware/EA and how we wound up in this mess to begin with, and also exactly when this ending was constructed and with what mentality was behind it. It's a do-nothing ending that leaves the series as open as possible for franchises, but also an ending/ending system intended to get people to spend more money on a game they already paid $60 for above and beyond any DLC. It takes a certain kind of greed to come up with the idea of finding ways to get people to pay more for the ending of a game they already paid money for.
As far as the spirit of the ending goes, all I can say is that I disagree entirely. If you've been paying attention, Mass Effect isn't really that "dark" or "gritty" a setting. It's about as upbeat and high fantasy as something can get. People seem to think that if you wander around a slum or run into shady characters this makes something dark and gritty, totally overlooking some of the places they went in "Star Wars" like say Mos Eisley where a guy got his arm hacked off by a laser sword. Even high fantasy involves jeopardy for the characters and good guys, sometimes very intense.
Mass Effect was defined as being a spiritual successor to "Star Wars: Knight Of The Old Republic" and going for the Star Wars vibe. The idea of the game is that while things are really bad, we have a bigger than life hero of the "Horatio Hornblower", "Honor Harrington", "James T Kirk" mould who is equal to those challenges and always manages to get things to turn up aces no matter how bad it is. Even Renegade Shepard is still basically doing the right thing for the right reasons, it's just how he goes about it.
The proper ending for Mass Effect is actually fairly predictable, that is to say that Shepard completes his mission and saves everyone, like he does at the end of the last two games. Yes, the galaxy is ravaged, but again understand this is a story with dramatic tension and indeed people are supposed to recover.
An ending montage of all the desicians Shepard made and how they pan out (which should be mostly positive in the spirit of this game) would also fit, though one as dark as "Fallout" wouldn't because it's a differant kind of game.
That's the kind of ending people would expect and accept, and I'd imagine how the game was intended to end to begin with going with the whole feel the series has had up until this point. That's not fanfiction, that's the concept that was created here.
Stories where the good guys lose have their place, but are generally an entirely differant in the spirit they are told from Mass Effect. It's also important to note that "The Empire Strikes Back" was the middle of a trilogy, an "Act 2" so to speak, setting up the jeopardy for the finale. While things ending on a dark note, it's important to note the heroes did walk away and were rallying, and almost from the beginning of "Return Of The Jedi" were setting things right, with the insane assault on Jabba's palace and sail barge fight scene right from the beginning (a situation where even with the jeopardy they seemed to be placed in they had apparently planned ahead for and had everything under control).
The only real "twist" to Star Wars was that it was Vader who took the emperor out, but that wasn't as big a twist as you might think because it was foreshadowed in Empire by Vader making it clear he loved his son and had thoughts of replacing the Emperor anyway. It's even less of a twist given the prequels if you know how prophecy works (ie it was time for balance, not a reign of evil), and sort of realized that as badly told as they were the story was kind of about Anakin/Vader to begin with.
In Mass Effect a twist like that wouldn't work because it was never written to plant the seeds for something like that. I suppose a surprise ending might have Shepard on the ropes, but having kept everything going until the last second so another character who had been there to begin with (Tali, Garrus, Ashley, or Kaiden... or maybe EDI since she was in the first game but I won't spoil that) can deliver the coup de grace.