Rare didn't make 3D platformers. Go play Banjo again. Practically no platforming in it. Even less in Conker. Rare made 2D platformers on SNES, but after that, they stopped making platformers completely.
That said, they still made good games, but collectathons are not necessarily platformers. I personally always preferred actual 3D platformers, like Mario 64, Crash Bandicoot, and Sonic Adventure 2. There are bad 3D platformers as well, I don't know how I could handle Croc's controls in a platformer back in the day, but I'd still like to see that genre make a comeback, and I fear that people thinking Yooka Laylee was trying to emulate 3D platformers, when it was actually trying to emulate collectathons, will make people think nobody should bother to bring actual 3D platformers back. The only hope now is that the Crash remasters are successful enough to warrant new Crash games, but it's Activision, so I really have no fucking hope for them being good, if they were to be made.
If you want to make a new 3D platformer, do it. You can have more larger, more complex levels. Some early PS2 games like Jak & Daxter and later Crash Twinsanity tried to bring the genre into open worlds without diluting the platforming too much (as open worlds tend to favor exploration over tightly controlled jumps and obstacles). Mario 3D World is basically the most recent actual 3D platformer (and focuses more on platforming than previous 3D Marios, too) and it's fucking great. It's not groundbreaking, but it just has good gameplay and good levels. That's all you need. Slap a shiner coat on it for graphicsfags and you're set.
This. Spyro did a better job merging collectathon and platforming, especially the first one. Better action in general, too. The faster pace and more frequent platforming (though still not enough that I'd call it an actual platformer) helped it feel less like just collecting things for the sake of it.
A proper collectathon still requires good level design. There's one way it is similar to a 3D platformer, or any other good genre, for that matter. Level design is always one of the most important things. Give me that and some decent controls and you've got a game. Everything else is extra.
3D platformers aren't about "walking around 'exploring,'" they're about walking around "platforming." Rare didn't make 3D platformers. Naughty Dog used to. Mario still does. Sonic Team sorta does. Rare never did.
'90s nostalgia is taking off after a near 20 year period of '80s nostalgia being dominant. They want one of the biggest genres of the '90s back. It's well known that 1998 was the peak year for video games, and 3D platformers were one of the biggest genres in 1998.
Of course N64 didn't have as many actual platformers (or games in general), so they had to make do with Rare collectathons. And they were pretty good too. So the promise of those coming back was good enough. But if that didn't work out, then I'll settle for the Crash remakes for now. Not that I'd actually buy them, but I'll hope they're successful enough to get some more actual 3D platformers made again. Bring back Croc. Bring back Jersey Devil. Do an ironic sequel to Bubsy 3D, I don't give a fuck anymore. They don't make genres I like anymore, I just want things to be like when I was a kid again. I'm sick of moviegames and I never bought into shooters or indieshit. I just want to run around and jump on things, but in 3D.
It was clearly just a technical limitation of the limited memory of a cartridge in the original games, so they shouldn't have done it in this game. The point is to make a spiritual sequel, and an actual sequel, if it were made even immediately after Tooie, wouldn't have done it. Hell, Conker didn't do it, and that basically is Banjo Threeie anyway.