Unfortunately the thing with Steam is, that at least some games bought there need Steam to be running to launch and work. As long as the old version of Steam keeps running that's fine, but incase there is any server related stuff, Valve probably won't care to break things once they stop to support it.Just keep old systems and old consoles around if you need to play vintage stuff. It's what gamers have been doing forever.
Not sure if the need for Steam is the case with all, esp. older titles. But at least some more recent titles like Tomb Raider or Hitman definitely ask for or launch themselves Steam when you try to directly launch the games from Finder. To my understanding they won't work without Steam.
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