A "life sucks" conclusion can work, but you probably don't need a long story to tell that tale. You can tell the story of "Life sucks, nothing makes sense, and you'll never know what happened" in fifteen or twenty minutes. If you stretch that out over an hour and a half movie and spring that on them at the end, then you should expect some people to be angry. If you take that message and spread it out over three movies, then you're a sadist. And if you spread that message out over three 30+ hour videogames, then you are going to end up with what we have here, which is people so frustrated and angry they will try to file an FTC complaint because they hate your art so bad.
If you are crazy enough to make a three-game series that ends on a black note with no questions (sensibly) answered and no sense of closure, then the least you can do is make sure the things you do say at the end doesn't contradict what has come before, or introduce new plot holes in the last minutes of the game. This is what Mass Effect did, and it's what has players so angry. Everyone dies, the bad guys turn out to be ridiculous, we don't find out what happens next, and the things we're shown don't make any sense. It is the most insufferably frustrating, obtuse, and nihilistic ending I've ever seen.
Having said all this, I really don't think this is an easy fix. Some people really did expect a mega-happy ending, and that's the only thing that will satisfy them. Some people wanted closure. Some wanted tons of possible endings. Some didn't care about the galaxy, they just wanted to retire on Rannoch with Tali. By saying they plan to change the ending, BioWare now has to decide which groups of people they're going to make happy. (All of them? Good luck with that.)
Are they planning to sort through the rat's nest of tangled lore they've cobbled together over the last five years and try to come up with coherent answers to everyone's nagging questions? Will they resolve the personal stories of your companions? Will they resolve all of the galactic conflicts between the various races? Will they let Shepard live? Will they give players all of the above, thus ending this gritty space opera with smiles and rainbows? No matter what they do, some people are still going to be mad.
If this really was the story BioWare wanted to tell, then they should stick to their guns and keep the existing ending. If this isn't the story they wanted, or if they didn't really have a story planned and were just making it up as they went along, then they need to fix that problem more than they need to fix the end to Mass Effect. Next time you embark on a five-year, three-game series with tie-in comics and spinoff novels, make sure you have some kind of plan.