Because I'm not convinced it would end there. Many of the people who feel GOG is inferior to Steam because of no auto patching, would then continue to say the same regarding cloud saves, achievements etc. Both auto patching and cloud saves are about convenience, are they not?
I see, yes those are matters of convenience, but the auto patching is probably the biggest one.
And as said, even if the "GOG client" had all the similar features and more, many people still would continue to say "no Steam key, no sale".
Well, in theory, if GOG did offer all of those things (auto patch, cloud saves, etc.), they would be offering many of the same features as Steam, and I think the reasoning people use in "No Steam, no sale" would effectively be neutralized. But I'm just guessing/theorizing.
But to bring this back since I think it's getting off track, I think that when GOG sells "newer" games they're mainly focusing on indies and such. Then, later on when larger titles are all updated and expanded with DLC, they could possibly come to GOG.
I think it's a matter of GOG playing to their strengths, rather than trying to outright compete with Steam.