I think you need to actually play something made in the past two decades instead of getting these absurd hot takes about an entire genre of games. "3D games may not have much substance, but there's certainly a lot of style and flair to them,"? "Well most of the ones after can barely be called platformers"? "2D platformers seem like they've become a timeless genre in of themselves, while 3D platformers come in and out of style"? The fuck?
Even the biggest, highest production value 2D Platformers with the longest playtimes and the most things to discover (and ignoring the Metroidvanias), your Super Mario Worlds and your S3&Ks and your Rondos and your Popful Mails, didn't have as much stuff as an average 3D one from just a few years later; and the overwhelming majority of them were not like that anyway. Certainly not in the wake Sonic left, when you had shit like Awesome Possum where the entire extent of the game's existence was copying Sonic's style and slapping whatever in after.
And platformers with combat being more important aspect of the gameplay than the platforming itself have existed functionally just as long. Ghosts 'N Goblins came out less than a week after the first Super Mario Brothers did.
And not every 3D platformer had the long term reception of the Sonic Adventure games and DK64, either. The reason that there are 500,000 generic interchangeable SNE-sque indie platformers on Steam is not because of inherent nostalgia/timelessness to a genre most game players nowadays probably never experienced to begin with. It's because they are cheap to make and can still be reasonably be made by one guy.
Also, this genre roulette crap that you've predicated most of your arguments on were things that most platformers had already stopped doing by the time the PS2 came out; because games (including Sonic Adventure and Spyro 3 and Donkey Kong 64) had already been getting shellacked for it in reviews by that point even when the games were still very well received. The collect-a-thon gameplay itself being the entirety of the experience is something that people were growing annoyed with in Rare platformers before Rare even stopped making them, and games that deliberately leaned into it heavily later on (like SM64 DS and Yooka Laylee) were torn apart for it when they came out. This is all shit that effort was being made into making it so there was more to the gameplay than jumping to a place to collect a thing so you can get to the next platform to collect the next thing even when Rare was putting 400 minigames and padding the game out with arbitrary character abilities in their games. Sometimes it actually unlocking new character abilities adding a Metroidvania elements to the level design or by focusing more heavily on action mechanics (or both); so the core gameplay loop itself was the rewarding part rather than just a means to collect more shit to pad the game out.
You're not describing 3D platformers as things you're taking umbrage with in this thread. You're describing the design ideas of the Sonic Adventure games and Donkey Kong 64 specifically did that have aged infamously poorly; and expanding the problems those games had onto the rest of the genre when they didn't even fully apply to their contemporaries. I didn't give two shits about the Big fishing levels or the DK arcade minigames when I was playing Spyro 2; and if I boot up AHiT or Crash 4 I'm not going to care that Yooka Laylee was dumb enough to bring them back.