a) Certified pre-owned and similar programs making some used cars more appealing (and thus more valuable), removing them from the supply of unvetted, cheap used cars.
b) More folks being able to afford used cars around the world. It seems like at the lower end of the market, many American used cars are shipped to Latin America or the Caribbean (in Europe many older cars are sent to Africa), again reducing the supply of used cars.
This is my sketchy recollection of things I've heard and read, I don't have any direct sources to link you to, but I've certainly heard multiple people bemoan that say, $5-10k doesn't buy you nearly as good a car as it used to (even adjusting for inflation).
EDIT: As an aside, the issue of shipping cars to different regions could also prove interesting for owners down the road. There are already some issues there - I believe my car's navigation system is for North America, or maybe US+Canada. So if my car gets shipped to Argentina in 10 years, the Nav will be useless or will need an update. Not a huge deal. But if Tesla's Autopilot is tuned for conditions and laws in a certain region, it would presumably require reprogramming for another region. Would that be an automatic over-the-air update? Or would Tesla not want to bother with free updates to a 20-year-old car?