That assumes that the used car market isn't a thing. Richer people (those who can afford new cars) keeping their cars as long as possible would definitely reduce overall demand for cars by killing the used car market, which I guess is also a good thing if you care about the environment (but in a harsh unintended consequence kind of way). Or perhaps new economy cars would become a thing again, kind of like the ones they drive around in China (or Japan). They aren't designed to last more than 4-6 years anyways.