The people buying $35K USD cars aren't likely thinking about selflessness, true.

But the question remains, if everyone who bought new cars kept them for 20 years and drove them a lot, what would the used car market look like?

Similar the rental market, I'd guess. Mostly done by those who couldn't afford to buy new but at a greater long-term cost.

I've seen people commenting that the money you save on buying new just goes on repairs instead: I don't agree. There's a fashion and entertainment value to driving something that works just as well as a new car but is newer, so the market gets pushed down.

The example of the Subaru Outback comes to mind: these are relatively harder to find used because most owners hang on to them.

~30% of cars are leased, so those would still be available