As soon as Steam decides to shut down (and it will one day, be it tomorrow, or 30 years from now, those games are no longer playable).
Nothing lasts forever.
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.
Almost every game has a pirated version in some torrent site.
To your argument I would say that Stadia is not the "Netflix for games". So I don't see why you expect to pay $10 and play any game you want like Netflix does.