> If GOG went down tomorrow, those games would be gone just the same as the Steam ones

Absolutely not. Libraries and archives are able to keep digital copies, with limits imposed on how they may share them, not on what they're allowed to keep. They're not allowed, without a convoluted exemption, to break DRM. So if Steam disappears, we're all collectively dependent on our ability to break DRM to maintain our culture accessible. Games that use draconian authentication could simply disappear. With GOG, the only thing stopping us from having easy access is our ability to walk to our local library.