Platformers did super well critically and commercially on the GameCube and especially the PS2 though. It’s more the eventual overexposure of the genre and developers’ shift to more "mature" games (read that as piss filter shooters)
I'm more of a traditional mascot platformer kind of person and the ones that came in the next gen were much more combat oriented or had some other subversion to it, and often relied more on egdyness. I understand there's people who's into those but they're not for me.
(Not that all platformers were like this, the best counter-example I can think of being Super Mario Sunshine)
Crash 4 and Spyro 4 failing sent the message that it wasn't good enough to keep making platfomers the old fashioned way, even though these games never had a chance being rushed and mishandled the way they were.
What you say is true and probably was inevitable either way, but I think it could have went a little better had the circumstances been different.