Look at it this way, assume humans drive, or need to be driven/travel, X miles per year.
Today, assuming a car drives Y miles per year, the market needs X/Y cars.
Given your argument, with self driving, a car runs nY miles per year, the demand for car will drop to X/nY, basically 1/n of before or maybe a little more, since after all the humans won't suddenly need to drive/travel nX miles per year.
So, if a car were to become worth 5x more, but we need 1/5x cars, the total market would remain the same.