That said, cool if pacman and AUR work for you, no need to change just for the sake of it!
I'd say Nix is even easier than pacman/AUR from a maintenance perspective because you can create overlays for whatever you'd like to change and not worry about the mechanics of building your own packages. The overrides are just maintained in the flake or nix config. Also there are already a huge amount of packages in the nixpkgs repo (probably more than base Arch + AUR.)
Arch is really great, but using the system now for ~5 years I cannot remember where every configuration is, which packages are installed and why, and things just get messy. I'm sure that some issues the system has are do to things I screwed up along the way, and these will be solved if I just reinstall. But with NixOS it feels so easy to find what changes you've done to the system, to leave a comment saying why etc. It feels to me much more maintainable.
i agree things get a bit messy but i try to keep the OS in a state where I can reinstall at any time without any trouble.
config files on github, pacman -S any tools i need, important docs all in the cloud already, couldnt get much simpler than that imo
i do get the desire to have a reproducible OS, just dont know that its something i need