See I'd agree with your argument if the savings were small, basically nobody is giving up their car for 10% savings, but these savings are very significant. Car ownership costs are the second biggest expense for most households after rent, and self-driving cars cut this in half.
I can see many households downsizing, and instead of owning 2-3 cars, only owning 1 and replacing those trips with self-driving rideshare fleet vehicles.
Especially for the younger generation - teens today drive far fewer miles than ever, which skews the costs away from car ownership even futher. I predict many will not get their own individual car, they'll just rideshare.