It's not just DRM, it's also an advertising profile and software usage monitoring application. Recording the number of hours I play a game or which game is about as invasive as when the Patriot Act tried to go after public library patron records.
GOG provides offline archive installers. A web browser can be used to both browse the GOG catalog and download updates. An app store's walled garden mimicking that behavior is not required.
Is it true that steam limits concurrent access to one's catalog, or prevents other family members from accessing the same account for a game if a different one is being played? Digital products should not be made to artificially suffer the limitations of real-world objects.
As for mods, some GOG games can't access the steam workshop recently. This is bad for the longetivity of the game and it's success, and of course for modders and players.
And I believe that there is still a steam runtime that games check for, even if launching outside of Steam. If I wanted random corporate software blobs running all the time on my computer, I would use a Mac or Wintel machine and call it a feature.