Either way, in many places (like San Francisco) the constraint isn't places that are too far away, it's places which have too much traffic between here and there. If you could replace a freeway full of human-directed cars with a freeway full of robot-directed cars, this would let you put more people not just further into rural areas, but around existing neighbor cities too. (One of the reasons the SF peninsula is interested in new "transit-oriented developments" near Caltrain instead of other developments: highway capacity on US-101 and 280.)

True, but the constraints to construction and therefore to densification - particularly in the Bay Area - are immense.

When it's hard to build in the city and it's the urban fringe is a miserably long commute away - housing prices go up. When it's hard to build in the city and the urban fringe suddenly looks feasible, because traffic moves faster - you see city expansion.

Agreed. Owning a car, let alone two or more for a family, is currently very difficult or prohibitively expensive in an urban setting. If you can have the same access as owning, but without the hassle, this removes a barrier for many people. In fact, car-sharing would likely have higher availability and therefore lower wait time in densely populated areas, in some ways increasing your "travel freedom" compared to an exurb.