People will ditch their secondary cars, but not their primary.  Right now, many families have two cars:  The family car, which is flexible, big and does lots of different things; and the commuter car, which is used to get to work and back, but sits at home on the weekends.  I can see people ditching their commuter car for self-driving, but it will take a long time for people to be willing to ditch that family car.