I think the big thing that turned fans off was the broken state in which the game shipped. I've seen horror story videos online where things are just straight up busted with graphics that don't render properly and generally garbage game play. They didn't help themselves any when they shoehorned the Kamala version of Ms. Marvel in there as the lead POV character. Yes, she's been in the comics universe for a while, but she's never been that well received and despite all evidence to that point, Marvel "keeps trying to make 'fetch' happen" with both her and other replacement heroes - and the reception among fans has generally been chilly. The same thing follows here - when it comes to video games, if you're making a game ostensibly based on the cinematic universe then you'd darned well better make sure that the MCU characters take the lead.
The game, overall, needed at least another six months (and preferably a year) in the cooker. That much is evident by the number of bugs present on launch. Combined with the GAAS model they were trying to take with the IP (and the lack of content in the pipe when the game rolled out), even a non-gamer could tell you that Avengers was going to be a non-starter. Now, I could be wrong. Crystal Dynamics could be hard at work and ready to roll out a massive content dump, but I highly doubt that's the case, and the game itself will turn out to go down in history as something similar, yet different - a big, steaming dump.