Dirty cars would be reported, so the previous user has to pay for the cleaning and the current user could get a free ride for the inconvenience. If people have to pay for cleaning then they will pay attention to keep the car clean.

Of course, the service is only a real alternative if a car is there within 5 minutes for you. People wouldn't tolerate more waiting.

It will be a gradual process and some people will still prefer to own a car, but in the long run owning a car could be a luxury if the driverless taxi really matures and it becomes much cheaper.