Except Valve could issue a final update that disables the DRM Check, and the game is still playable locally, right? More likely than Stadia developing a local client to sync game content/licenses to.
Also, having worked in game sales, it turns out that buying "drm free unlocks of long tail" games is not expensive. How do you think things like PS Plus Free Games works? Essentially game devs book revenue now for possibl future revenue lost. It's a smart trade for most games unless that game is say skyrim.
Also, PS Plus games are still DRM'ed, and you only have access as long as your subscription is valid.