At what point can we just return to 1990s wheel sizes, add the motor to the wheel, and have the same unsprung mass as we have with today's gigantic wheels?
As soon as the roads are as smooth as they were in the 90s. I have a pet theory that wheels have gotten huge partly in response to deteriorating roads - larger diameter means less leverage against the suspension when hitting defects. It’s the same reason dirt bikes have large front wheels.
The aspect ratio (sidewall height) of tires has decreased in a lot of vehicles though. They are driving around with low profile rubber bands on the rims. Looks cool, but not much fun in the potholed mid-west.