this is a different problem set from Anthem. Where Anthem had everything into multiplayer from the start - Avengers didn't even start multiplayer until late in the development
The SP is fine. But one of the biggest things that bodied Avengers was that you literally couldn't connect to other players for weeks unless they were already on your friend's list. There's no way to kill an online population faster than having no functional matchmaking. Then when they did get it working, all the end game content worth doing was to be done solo. And further, you can't even have duplicate heros in the same party so shyt is inherently fractured there as well. If someone wants to play nothing but x character, or x character is more popular than others, people will have harder times finding games.
Monstrously dumb
The SP campaign is aite. The solo multiplayer/post-game experience ain't bad either. Its just easy to do everything and once you're done you're done. There's no raid, no boss rush, no endless horde mode, no pvp or pvpve, no real functional hub that forces player interaction and competitive measuring, no stat trackers, clans, etc
all that shyt was an afterthought. and it wouldn't even be hard to slap that shyt in there with some new enemies right around the next big movie drop and bring people right back either since its not subscription based