Even if you assume requiring human takeover to be a relatively uncommon event (whatever that means), as soon as you posit it as something that will be needed from time to time, you've significantly constrained the car's usage models. You now must have a licensed, unimpaired driver in the car at all times. Even if they don't have to be paying attention, this means no empty cars, no unaccompanied children, no "driving" home from the night out, etc.

Sure, but maybe we can start there. I would certainly buy a car that could drive itself a significant percentage of the time

Oh, I would too assuming it were relatively affordable. I'd be pretty happy with one that even just let me doze off when highway driving in a limited set of weather conditions.

I was just pointing out that, if you can't guarantee you won't need to handoff to a physically present driver, then there are a lot of things you can't do with the car even if needed interventions are just an occasional thing.