The MCU has run out of steam after they got away from the A list heroes. It's not that B- or C- list characters can't do well, it's just that it takes a talented writer/director to do it with a good movie, and Disney/Marvel just isn't capable of getting that consistently on their own, they're too safe & corporate. They're just trying to recreate what made the original MCU run successful, but we've already seen that. I don't know that there's necessarily "superhero fatigue", though trends in movies & pop culture come and go, that's inevitable, but for me it's more been "bad writing fatigue", I'm just sick of their lazy ass shit writing, it's insulting and I'm tired of being treated like that by the MCU.
I'm not sure how much credit Disney should get for that compared to James Gunn, considering they fired him and he was able to turn Suicide Squad into a hit as a result.
When the MCU started I don’t think people considered Iron Man, Captain America, Thor as A list. They were just the properties they weren’t able to sell off yet. Doing these movies was a big gamble. They needed to succeed so they put their best effort into them and it paid off.