With a car, they slap a transmission in it at the factory, and you go drive 100,000 miles with it. They put brakes in at the factory, you replace pads every 50,000 or however many miles.
With a bicycle, brakes and transmission seem to require constant maintenance, lubrication, adjustment, etc.
So my opinion, stuff works fine as it is, but the biggest improvement would be getting stuff where it just doesn't need to be adjusted or maintained to work right.
Coaster brakes and geared hubs were kind of a step in that direction, but they have their own issues.

On my Raleigh Sojourn, with 9-speed drivetrain, more often than not, I'll be shifting up or down 2 gears at a time anyway. So the "continuously variable" doesn't do anything for em.