I don't know about the overall market analysis so I'm not commenting on that. But I do know that personally, GOG meets my needs in spades and Valve doesn't, so I've spent a lot of money at GOG and will for the foreseeable future. I am a Linux guy and don't own any Windows machines. I couldn't care less about the Galaxy client. To be honest, I'm not even sure what it does -- I just know that I get along just fine without it.
the unification of other game stores is just largely broken. They haven't maintained their plugins for many years and as the stores change, functionality breaks in weird ways.
For me, GOG is as close to perfect as I've found.