I'm a huge cheerleader for GOG, and would be on DRM grounds alone, but they also provide a good shopping experience and so on. I am philosophically opposed to Steam and other DRM-saddled sources, so I have less experience with them (very little with Steam, none with the others), so this article was (as usual!) quite interesting.
I noticed you had nothing to say about Humble. As a customer there's some things I like about it (DRM-free titles are marked, torrent downloads are available, and no registration is even required), but Humble seems to have a severe packaging problem: downloading one's games as a ZIP of loose files is fine for a technically-minded person like myself, but I can see how others might be daunted.
As far as GOG's curation goes, I've often wondered if, were it a free-for-all (or near to it) we would get a significantly larger slate of games there. I've heard stories of studios wanting to get on GOG but not being able to, and I've wondered how common that really is. I'm sure you can't answer that; I'm just thinking aloud. The more quality games GOG has, I feel, the better it is for everyone: customers, studios, publishers, and the industry at large (though GOG may feel differently).
I've never visited Itch.io before; now may be a good time to start.