Just a thought here, and sorry if it was mentioned, I speed read, but if GOG started to carry AAA games (other than the ones they already do) isn't it probable that many of those titles will start to carry DRM of its own? And if that happened, wouldn't that defeat the DRM-free purpose that is one of its main driving forces?
I'm not a huge fan of steam, however I have about $3k worth of games on my client. I like having my games in one place, having them auto-updated (and yes I understand patches can break things, but luckily this has only happened to me once or twice) and a save cloud. Steam has become slightly more relaxed in its TOS over the years (they used to ban your account if you had a fraud charge on it, now they just prevent you from purchasing anything else) and I'm not worried about them "going anywhere" or "flipping a switch."
On-topic: This hidden achieve thing is most likely something for halloween, and even if it pertained to something game-wise (which achieves don't count for anything so its not like GOG customers are missing out there) I am 110% positive that CDP (CDPR?) would patch that into all copies of the game. (minus the exclusive steam achieve things)