As soon as they lost Iron Man and Captain America after Endgame's massive peak, the only way was down from there.
I wonder if Marvel's business model and strategy of staying as a main interest was annihilated by Covid. Think about it, people had been getting 4 movies a year or whatever for over half a decade. Then boom, everyone goes cold turkey for 18 months or whatever.
A lot of people just lost interest of that?
but yeah, the repetitive formula being overplayed for too long alongside of the loss of the main characters and also some mediocre movies was the killer