Without leases, I'm not sure that there would actually be any supply for the used car market. There are some small percentage of people that buy and sell new cars every two years, but most people I've seen buy cars and drive them into the ground until they hit a major, expensive mechanical repair or couldn't get them to pass inspection any longer.

Most used cars I've looked at in dealerships (as opposed to the Craigslist/Uncle Henry's as-is-where-is private sales) were overwhelmingly former lease vehicles.