People will still be doing about the same number of miles per year, and cars will still last a similar number of miles. So if a ride share car does 10x as many miles per year we need 1/10 the cars, but they also last 1/10 as long, so it evens out.

Sure they'll get slightly more miles out of a ride share car, but the number of miles will also go up do to dead heading and because cheaper/better transportation causes prior to use more of it.