Oh look I've even seen this same opinion in the, past, at least film critic hulk managed to make his elitism mildly entertaining to read, this doesn't even manage that.
The ending itself was a perfect storm of fuckups, just eliminating the downer ending wouldn't have satisfied more than a few people and we still would have likely gotten the same controversy we got now. The ME3 clusterfuck was a massive combination of factors and reasons that just reached the boiling point and everyone went wild. You would have to eliminate a significant chunk of those factors before the game came out, to stop the backlash of what happened.
1. Pre-release press statements built the ending up specifically to not be a simple multiple choice A,B,C ending, but it turned out the actual ending was exactly that, they didn't even really try to dress it up or hide it. It was literally shepard standing in front of various epilogue cutscene buttons and they didn't even have the integrity to not reuse massive amounts of assets so the endings almost look like color swaps of each other.
2. Plotholes galore! There were a bunch and I won't waste the time pointing them all out since others have done it far better than me over a year ago. The big one of course is Joker gathering your squadmates offscreen and making it through a relay all before the explosion hits, even if you can justify it, it's still basically cheap emotional manipulation meant to make you worry about your crewmates. The extended ending improved this flaw the most, but it was too little too late at that point Bioware wasn't putting the genie back in the bottle.
3. EMS was pointless. Seriously EMS was completely pointless the only thing it changed was who walks out of your ship at the crash site and whether earth got decimated, it also changed what ending options you got, but none of these changes made any sense. The entire assault on earth, the culmination of 3 games worth of plot and action was just a linear corridor in a fairly ugly setting compared to the other setpieces in the game, it was pretty much just slogging through enemies and was all pretty much the same no matter how high or low your EMS was. This is pretty much unacceptable after ME 2's well-executed suicide mission, so we know Bioware can make an interesting final mission where your choices can effect things, but they still fucked up the entire final chapter of the journey.
4. After Dragon's Age 2, there was already a contingent of fans out there ready to leap on any Bioware screwup. And there were some good things to criticize before you even got to the ending. The simplified dialogue wheel, the elimination of any sort of vehicle component, again. Planet scanning somehow managed to be even more annoying than Mass Effect 2. Kai Leng couldn't hold a candle to Saren and Harbinger as far as villains go, he actually seemed to be a step down in threat from the villains of the previous 2 games. Most of this stuff got drowned out by ending rage, but it still added fuel to the fire.
5. It tried to be sad and failed. Look I don't mind sad endings, but only if they are sad for a reason, and not just because everybody dies in the last 10 seconds to a giant energy wave. If the reapers had been shown tearing apart the quarian or alliance fleets that would have been sad and justified within the setting, but what we got was a giant energy wave that strands a bunch of species on one end of the galaxy, most of them with no chance of ever seeing home again, and strands the Normandy on an alien planet, also with no indication of them ever getting home again. It's a contrived sad that comes out of nowhere and feels like a, "rocks fall, everybody dies" kind of ending.
6. the Coup de Grace to all of this was that when the game originally released, literally the last thing you saw before being kicked back to the title screen was a pop-up box that said, "Thanks for playing, now go buy some DLC". I'd imagine that was like a focusing lens for a lot of the hate that got spewed right after. That little corporate message probably galvanized some of the EA hate we see even today.
People have a lot of reasons for hating the ending, and fixing only one issue (making a happy ending) would only satisfy a very small number of the people who raged against the game.