I can't remember exactly who said it or even what was exactly said, but the idea was that there were four stages to fads.
The first stage begins when the first movies are made. They're considered long-shots, don't exactly have a lot of star power, and usually get produced on humble budgets. If these projects don't get any attention, then the fad ends then and there. For geek culture, this was the early movies all the way back to the late 90s. X-Men, and Raimi's Spider-Man, eventually getting into Nolan's Batman trilogy (though that one was a bit of an outlier) and the very first MCU movies like Iron Man and that Hulk movie nobody remembers. Sure X-Men had Patrick Stewart, Ian McKellen, and Hugh Jackman but really, these people didn't become really big until after X-Men. Stewart was popular with Trekkies and you didn't know McKellen unless you were a fan of English theater. Downey Jr. was considered a junkie washout before he landed the Iron Man role.
If the fad is successful, it moves into the second stage. This is the moment when the fad really become big, to the point where other fads get placed in the dust. This is the moment when you go all-out on true star power and over 100+ million dollar budgets. The moment geek culture truly reached this phase was in 2011-2012, with the release of Avengers and the conclusion of Nolan's Batman trilogy. It was indisputable that geek culture reigned supreme at the box office and pretty much every studio tried to copy the MCU formula, none of which succeeded (not even other Disney properties!) However, eventually, people do start to get fatigued, and more importantly, the very creators of these products start to get fatigued. I think that the time for this stage was from the first Avengers movie to Endgame.
Which leads us to the third stage: absolute pretentiousness. This is when the people making these billion dollar movies start to get discredited by more serious filmakers, and despite the studios swimming on piles of gold Scrooge McDuck style, they start to get a chip on their shoulder because the Oscar winners start to hurl criticism (whether you think the criticism is fair or not is up to you). Therefore, the studios decided to get some "real" filmaking credit by hiring a bunch of auteurs. This is when you start getting the Rian Johnsons. Even if you liked the movie, Joker is probably the best example for the "subversive" stage and I fully think that we're entering it now. Even if Marvel will not change a bit, the fact that Feige is now trying to sell how different MCU Phase 4 is going to be really tells you the state of things. From trying to introduce sitcoms in WandaVision or saying that Doctor Strange 2 will be a horror story.
The fourth stage is decline. We have not hit it yet because I think we're still the afterglow of the second stage and entering the third stage, but it will happen eventually, regardless of how good Feige is at maintaining consistency.