Unless and until the US and European population is willing to give up car ownership wholesale....

That is the theory, as I understand it. Folks like ARK believe that pure AV TaaS will be so much cheaper than operating a privately owned car that people will give up car ownership wholesale. They think that these things will be so much cheaper that people will make that switch.

Personally, I'm highly skeptical of that. The main costs of owning a privately owned car are the costs of operating the car: depreciation, maintenance, fuel, insurance, and repairs. It costs just as much to operate a privately owned car for (say) 15K of miles in a year as it would to operate a TaaS car for those same 15K miles. TaaS vehicles save money on the costs that accrue when the car isn't being used - almost entirely financing and parking costs. But those aren't going to be enough to fundamentally reduce the overall cost per mile for anyone other than infrequent drivers, especially since TaaS cars have their own expenses that privately owned cars do not (like deadheading and the monetized cost of cleaning and upkeep).

The real question is whether TaaS gets cheap enough to be a solid "second car" replacement. It will likely never be materially cheaper than a privately car that's driven 10-15K miles per year - but it might be cheaper than a very lightly driven car.