The hardest thing for anyone to do is suck it up and move on.
I speak from similar experience. I've GM'ed roleplaying games for several decades, now, and I've had times when I've either scripted together what I thought was a really fun experience, or a really epic combat event... And everyone looks at me when I drop it on them like I've suddenly grown two heads. They get upset, because I'm disregarding something they wanted, or I tried to shoehorn what was going on into something I wanted to do.
When this happens, the first instinct is to dig in the heels and force through it. After all, you get emotionally involved in what you created. And you truly believe you built something interesting.
The difference, I think, between me and a lot of what we've been hearing from Bioware, is that I've forced myself to step back and ask myself if my position was really more important than the whole game. If the story I was telling was that important to me, a screwed-up chapter that was alienating my players wasn't worth keeping going. Sometimes I had to take 5 while I rewrote a few things, or sometimes I was able to patch something together that used most of what I'd written up. But the key is to recognize that the whole story is more important than the one disagreement. They don't get up and walk out because you made a bad call. They'll respect you a hell of a lot more if you can accept that your vision wasn't as cool for your players as you thought it would be, and rewrite it to reflect what they wanted.
I think that's what Bioware has to do. They have to stop looking at the endgame furor, sit back, and consider the whole 3-game story they've been telling. Is that story more important than fighting to keep an ending your players aren't happy with? I'd say so. I'm biased, of course, but in this case, I'm one of the players who wants the GM to change the scene I'm very opposed to. I'm not going to storm off in a huff, if I think this can be resolved. And if it is resolved, I'm going to still be interested in any future games the GM wants to put together, because this one event is just that: It's a one-off. It's not like it's what this person is like all the time.
So that's my two cents' worth. If they haven't already, they should step back from the topic, and spend some time recalling the whole story. Is that more important than the fact that the players don't agree with the GM on what constitutes an "epic" ending? Again, I think that it is. That said, the key is to accept that the players and the GM have a disconnect that needs to be patched. Then fix it, and move on. There's plenty more story to tell, and what needs to be patched can be made epic.
Idea:
For the "epic" ending, try this. Add an option to the Options screen, which allows the player to select the "original" Mass Effect 3 endings, or the modified "Epic" ending. I'm not sure how you'd tie totally different endings into future DLC, but that way, you really can leave the endgame experience to the player, and add a level of choice into the game that will appease everyone. Even if the Epic endgame is as much on rails as the original one, just being able to choose between them adds a level of metagaming choice, which would be pretty unique. And it means that for those who like the existing endings as they are, nothing needs change for them.