That assume almost everyone using autopilot will replace that with manual driving in the same car. Some of them might get cheaper cars or take uber and end up less safe.

And even then, there's lots of stuff you can do to a car that makes it less safe that we don't ban, as long as it's still sufficiently safe.

If anything, increase safety standards. If you want to do something that won't cost extra money then do something to ban oversized cars at the same time...