Used cars are not supply-constrained. Inventory is declining only because new car supply has been so suppressed over the past couple years, but that doesn't mean they're running out any time soon. So the opportunity cost on the individual level is as I describe: do you reutilize an existing car or do you in
carnate (heh) a new one with your market participation? It's true that your new car will likely be someone else's used car, but the argument has a whiff of trickle-down-style economic reasoning that would take more space to unpack.
Further, more demand for used cars would spur the industry to reclaim/refurb salvage titles that were otherwise destined for the scrap heap and sell them on.
We could easily do this wrong, e.g. by shipping busted cars overseas, having them refurbed there, and shipping them back. Or we could do it right and create jobs in America that would stimulate economic activity locally.