People like John Campea were predicting superhero fatigue in 2017 after Justice League failed at the box office. But instead superhero films - or more specifically, the MCU ones - have just gotten bigger and bigger, and in unlikely corners like Black Panther and Captain Marvel, both entering the billion dollar grossing club. The past two Avengers movies have redefined the word epic, in space and audience appeal.
The only thing Justice League proved - just like Transformers: The Last Knight that same year - is that the audience only comes so many times to a mediocre product before they quit on it.
As for the MCU getting even more experimental in their next phase, I say they have always been experimental, making even the most unlikely property get embraced by the paying audience.
This constant prediction that the audience is getting superhero movie fatigue, it's like the NFL experts who've been saying for years this is the year Tom Brady and the New England Patriots fall off a cliff. You say the same thing for years and it doesn't happen, you lose credibility in my eyes.