Probably a vocal minority here, but I think that YouTubers like Jacksepticeye switching to playing mostly triple-A games instead of totally random and silly games that would have otherwise faded into obscurity was the point where YouTube started to show cracks and go down. The "silly" era of YouTube can't be beat.
VanossGaming is another one, to an extent. He's still fun to watch, but his videos today aren't nearly as creative, often blatantly following trends and repeating the same jokes over and over (Mini Ladd being a pedo, them being "irrelevant," how Vanoss carries the group, etc.). It's gotten to the point where I just know that when GTA 6 Online comes out, he's probably going to put out a dozen videos of it within the first month, much like when Wildcat went on a huge Fortnite binge in 2019.
And of course, you can't forget folks like Lazarbeam who completely sold out after becoming successful, then fell off the face of the earth a few years later. His BeamNG videos from way back when he was fairly popular, but not huge, remain classics.