I agree.
Given we want to merge the 2 domains into one (which we do), I would vote for nix.dev to be merged into docs.nixos.org as suggested by @Infinisil over at the mentioned GitHub issue. And retain nixos.org as the single canonical domain referring to any official resources. This is also what we might be progressing towards given this comment. This probably should also be reflected in the RFC regarding this if the RFC is going to be the path this domain change takes shape.
TL;DR: My rationale is that Nix is quite coupled with NixOS (at the development end), and NixOS includes Nix, not the other way around. And, using nix.dev will only further stagger the popularity and/or adoption of NixOS.
If I as a naive user were to go onto nix.dev, I’d have a good chance of never knowing about NixOS if I ended up not using Nix and only exploring it. Whereas if I visited nixos.org, I immediately know that NixOS is an OS and it uses Nix. Here, I’ll eventually learn that Nix can be used standalone as well. Even though this is a minor thing and that any user going into a good enough depth of Nix would learn about NixOS, but I think this is a point worth considering.
The nixos.org webpage also already teaches the user about Nix: Learn Nix | Nix & NixOS