I feel like it's all tea leaves divining anyway. In a way The Marvels is the most Avengers like movie in all of phase four, with properties from two different shows and previous movies coming together to fight a big bad, and it does the most poorly.
Maybe it was bad luck that the clear overarching storylines were introduced in Eternals and Quantumania, the two most panned MCU movies (though personally I found Eternals probably the only worthwhile movie in phase four), maybe it was arrogance that made them introduce these in lesser movies to sell more tickets for them. Either way I think it was misguided to think that people showed up for all these MCU movies because they were connected "somehow". Fans are salivating about the teased hero in the after after credits movie! Ultra nerds were, most people didn't care, what they did care for was a narrative arc that continued on into the next movie. That's something that's really lacking. Quantumania and The Marvels had this the most, but at the same time, Ms Marvel and Captain Marvel didn't really carry anything with them into this movie. There's no reason to watch this movie, outside of really liking the characters. Which I do, but not enough to buy a theater ticket.
Speaking of really liking the characters, the most criminal thing was not following up on Hawkeye+Yelena.
Anyway, it's probably also just superhero fatigue and too many weak movies.