"Over-estimated their fans"? Nice bit of arrogance there. There is nothing inherently 'better' with depressing or bittersweet endings. They're just endings just like any others, fitting to some story-arcs and not fitting to others. It's certainly one of the ending types that can work for a game like this but it sure as hell doesn't work as the only possible ending.
For a huge amount of the players out there, Mass Effect has always been about the hope against the odds. Triumph against the impossible. I don't know if any of you remember that moment when you first finished Mass Effect. When everything came to a peak and you finally took out Saren/Sovereign. Explosions, the caught breath when the tower gets hit by a piece of Sovereign, the resignation on your team-mates and Anderson and so on. Then that blink of motion, Anderson turning, then Shepard shown climbing wounded but alive on top of the defeated enemy with a cheeky smile. Reading the forums and remembering my own experiences at the time, it was the moment when fists were pumping in the air, whoops and cheers filled living rooms and computer dens alike. Hell, the time spent just staring at the credits with the amazing music going, getting strength back in your knees and the time needed to get down from the high.
That is what Mass Effect is to a staggering amount of players, and there is nothing wrong with that. I'll admit it myself, I loved that feeling and the victory. The reward for perseverence. The pay-off for all that effort. The win. You can call it a 'fairy-tale ending' as much as you want, but the derision inherent in that is misplaced and frankly rather ignorant. Not to mention the fact that you have to keep in mind the medium as well.
Video games differ heavily from movies, books and TV series and so on. It differs because the players put so much of themselves into it, games like Mass Effect and other RPGs in particular. There's so much time and effort spent. When it all comes to a head and it turns out there's no reward at the end, no pay-off for the hard work... just a loss, no matter what you do... it's not good enough. Three whole games worth of gameplay and there's absolutely no way to win? No way to beat it? How the hell are people supposed to react to that?
This series deserves so much better and so do the players. Of course the bittersweet and bad endings should be kept for those who like that kind of thing and for those who's playthroughs reflect that, but a lot of people played these games to win. A lot of people played this game to get a good ending. How about a middling good/bad ending? A really bad ending where the galaxy goes poof?
There is absolutely no excuse for locking all these players into one single utterly shit ending. Especially when there's absolutely no answers given as to what the hell happens to the galaxy and the races in it, plot holes not even mentioned. The plot holes alone are terribad enough.