It will start in cities with people declining to purchase new cars since they can just take a cheap self-driving car everywhere. It's true that at sufficiently low population density it would not makes sense to rely on a taxi service even if most people were using it, but I think that is only in very rural areas. 90% of the Americans live in an region where, if they all used a self-driving taxi service, the experience would be better than everyone owning a car. The hard economic benefits of not having to worry about maintenance, parking, or the up-front capital costs at purchase (huge!) will steam roll the romantic aspects of owning a personal car.