CD Projekt Red who own GOG naturally distribute their own games through GOG, but note that they as well distribute them through Steam. I see no clear reasons for Valve to restrict the distribution of their games to Steam only. Yes, historically it came with this not very invasive DRM, but there is still no reason to keep it forever, since DRM has no good reasons to be used altogether.

I understand that Valve can be already too involved with game producing companies which still have backwards thinking that DRM is a must. But Valve can start pushing them to drop it, instead of being passive.