I feel like this is a good argument for drm-free games and stores like GOG. Not that you as a consumer can always choose that, as many games don’t offer that option, but for the ones that do, there’s less barriers towards playing it in the future or in environments where it’s not originally intended.

There is steamcmd, an official command-line tool— I’ve only used it for game servers, and I don’t know if it includes the Steam runtime/resources, but I know it lets you download games.

You could look at Goldberg Emulator too. I know it’s used often for piracy, but idk about its legality on its own.