But nobody was told "you have five years to trade your horse in on a car, or send it to the glue factory". Horse are still legal and used to this day, they just mostly lost out to market forces... that is, cars are simply a better mode of transportation for most people most of the time.

But you can still ride a horse if you want to (albeit probably not on freeways, but certainly on country roads). Now, if the proposal was that certain limited access, high-speed highways are restricted to only self-driving cars, I might be able to accept that, even if somewhat grudgingly. But outlawing human driven cars completely, even on backroads? I just can't see it.