You appear to be making an unfounded assumption that regardless of how many owners it has, any given vehicle will remain on the road the exact same number of years.

If an owner decides to keep and maintain their car longer than they would have in the past, that car will stay out of the junkyard longer and therefore reduce the need to produce new cars. They will use some of the money they would have spent on a new car to keep the old one going longer, and still pocket the rest of it as savings.