Besides what the other person said, there's also the whole treating Linux users as second class citizens. If they didn't had a launcher for Windows, then it wouldn't be that big of a problem, but the fact that they did created a launcher for Windows years ago and porting it to Linux has been the most upvoted feature request since then and they haven't done it is a slap in the face of a community that shares a lot of their beliefs. Valve is investing money on making Linux gaming a reality, GoG won't even port their launcher to Linux, despite not caring for a launcher I know who I'm giving my money.

I like having all my games in one place, on a platform where Linux “just works” and I don’t have to fuck around with it.

Eliminating third-party launchers sounds great in theory until you have 20 different half-baked second party launchers that serve no purpose other than being a barrier between me and the games.

You may not agree, but some people actually like the platform integration features that Galaxy and Steam and the like provide. Cloud sync and achievements and things that you may not care about are important to other people.

And then there's just the whole "They said they would, and this is not very reassuring about their commitment to Linux users."

If third party launchers were as good as the first party ones, we wouldn't.