Good question, and for many it will not be, and rentals are acceptable.
But also for many, renting a car has a huge ICK factor. It is one thing while traveling to rent from an agency who has (purportedly) thoroughly cleaned and inspected the car before you get it. It would be quite another to rent cars like scooters, where the previous user likely smoked, left wrappers and food debris, and who knows what else, even damage. Plus, most people who own cars keep a fair amount of stuff in the car for their specific convenience, and have their own settings, etc.
The fact that the likes of Zipcar, Turo, and the lot have not entirely taken over urban transport but instead remain niche players shows the extent of this preference.
For suburban and rural markets, it just gets more extreme. How quickly could a rental service be able to deliver a car; could it reliably do it in less than 5-10 minutes for people to run an errand? If not, unless they are insanely cheap, ppl will likely want to own their own. Perhaps it'll be more of a hybrid, households owning one car and renting the spare for specific trips?
To take off as a real replacement for ownership, self driving cars likely needs to meet at least these criteria: 1) overall cheaper vs ownership for average mileage/year, maybe 12kmiles/yr. 2) consistent delivery of car to rider in <5-10min, 3) a way to ensure the cars are always clean when they arrive (how? route them all through a cleaning station first?/lotsa cameras in the car to monitor cleanliness?/ability to order replacement car in <3min?).
Seems the short rides
, if they are cheaper than ownership, .
A lot of folks are relearning lessons on this front in Cloud right now.