I can think of a couple of valid reasons someone would like to use an officially supported client, like inherent trust, ease of use, and a soft guarantee for support. But for people you're calling supremely anti-corporation and anti-closed source it doesn't make sense to use either. Both are owned by firms and are closed source. And if you count Steam games requiring the Steam client to run, then Steam also has DRM on all games

Using GOG Galaxy also won't fix the game related issues people have with Heroic. Heroic pulls the same files Galaxy on Linux would. Hence if game doesn't work, it would likely be a Wine or Proton specific issue. Heroic also automatically applies known fixes to games using Proton. So I don't know what Galaxy could do to provide a more stable experience.

Edit: missing word (use)