Ubiquity and response time matter. I live in Los Feliz, a popular LA neighborhood full of lots of 20 and 30 somethings with disposable income. It's nigh impossible to hail a cab here. In fact, outside of NY and the downtown business districts of a few other major American cities at certain times of day, "cab on demand" basically doesn't exist. If I want to go out without driving on Friday night, I use an app like Taxi Magic to request a cab, then wait somewhere between 10 and 30 minutes for a cab driver who speaks relatively poor English, has never heard of the bar I'm going to, needs me to provide turn-by-turn directions the entire way, misses our side-street repeatedly, and then is annoyed that I'm paying with a credit card.
Imagine instead being able to use an app where, in any urban area, you can have a car sitting in front of your house in 3-5 minutes, with reasonable mileage rates, exact GPS coordinates preprogrammed, and no haggling over tip and payment method at the end of the ride.
Speaking as half a couple paying $1000 a month in car payments, car insurance, maintenance and gas, this is going to fundamentally change the calculus for private car ownership in America.