[…] because they don’t allow you to run older versions of games.

They do if the dev makes it available, I’m looking at four different versions of Terraria in the beta menu right now that stretch back four major versions. I’m pretty sure a couple games in my library somewhere have their entire update history in there, though I can’t think of one to name off the top of my head right now, that’s not a feature I use very often.

This is not true of all games, but it could be, either directly by game devs without Valve even having to care, or via pressure by Valve by just making older versions available whether the devs want it or not. I think the latter option is probably the better move, but there’s technically nothing stopping the former other than the game devs themselves.

There’s also a valid argument that making downpatching very easy would be a huge boon to piracy. This is a reasonable talking point no matter which side of that fence you sit on. It would also probably benefit modding as well, which I think is a more objective good but some game developers or more likely publishers would probably disagree.