I fully agree. If you can hail a robot at the price of driving using a car sharing service (as in short term rental, not as in "ride sharing"), the threshold to carless will fall a considerable bit. A lot more people are in the nearly-carless-but camp than actually carless.

Current car sharing schemes only really work for short two way trips without much of a stay at the destination, since you would block the car for the whole duration. Robot taxis (just like their conventional counterparts) on the other hand could be booked separately for each direction,taking the duration of the stay out of the equation (except when it is so short that it is cheaper to keep the taxi waiting). Once on this level, it's just natural to replace the easy trips with public transportation.

In the dense cities where this matters most, people already don't own cars.

Sure, but the thought is more that it makes it easier to live in marginal car-less areas (which should then attract the investment to make them less marginal).