If it is a self-driving software update that the manufacturer could push but chooses not to (or could trivially port), I think it becomes much more difficult liability-wise and legally for them. I'm not saying that is the correct way, but I think it is how it would work in practice.

We do in some places. Where I live (Iowa) we don't - but most people take better care of their car than that so it is pointless. As a kid I remember parts of MN requiring inspection and a few years latter dropping it when they realized nearly every car was passing so there was no point (this was emissions only not safety). In Texas there are regular inspections - but if you go to border towns you see a lot of those poorly maintained cars on the road (despite the inspection) and so people see more need for them and they keep them.

See this is why people in iowa don't deserve a senator