Not quite as much of a coincidence as you might think. Many of these algorithms come from graphics researchers presenting at SIGGRAPH (https://www.siggraph.org, the leading conference).
So if so if Jet Set Radio was released June 2000, you can look for related papers a couple years before to see if new techniques were appearing. And, in fact, they were!
Disney paper (1998) on texture mapping for cell shading (the color of a cartoon):
Interesting. There actually are various recent hobbyists who have achieved these effects even on an N64 due to the quite programmable hardware. But if the basic idea or concrete algorithms were only invented in 1998, it makes sense that contemporary games didn't use it until the Dreamcast was out.
The N64 was also the most powerful console in its generation, so really it’s the next most powerful before the Dreamcast.
It only looked like crap in its era because carts were expensive compared to CDs. Which is less of an issue now.
Also the hardware antialiasing and overuse of fog didn’t help its case. Thankfully the former can be fixed either via hardware mods or emulation.
I’d be still be interested to see if those demos you saw were full games or not. I’ve seen a lot of cool effects in games get abandoned because they didn’t scale well to a fully fleshed out game.
