Apart from the fact that all things you listed exist in one form or another on GOG, have you considered that there are people that prefer not having all those random bullshit social features in their game store? Stream may be more popular, but getting out of their niche and copying steam will alienate their fans without attracting steam users.

Also fuck the workshop, it’s the worst thing that ever happened to modding. It’s a total pain in the ass to download and apply a mod that is only available there to any non-steam version. Additionally it produced a generation of gamers that is unable to trouble-shoot even the tiniest problem with the mods they applied.