I'm including Uber and Lyft in "taxi", i.e., driving service on demand. Replacing a personal car with Uber and Lyft is too expensive for most people, and the basic economic reason is that a human driver must be paid. But in a few years self-driving taxis will be cheaper than taxis with human drivers. At that point, a larger and larger share of the population will stop buying cars and will just take self-driving taxis.