Some people can't seem to see why Nix and Guix are in an entirely different class from traditional package managers, and I don't really want to get into that fight, anymore than I want to get into a fight over whether Rust is better than C++. But for what it's worth, installing and using Nix inside of Arch Linux is not particularly dangerous at all. Nix itself is in the Arch Linux extras repo and can be installed directly through pacman, and it mostly writes to /nix/store so it is unlikely to deal serious damage your Arch install as far as I am aware, or have experienced. The fact that Nix can be used outside of NixOS is relatively important, in fact, since a large amount of Nix users are using it on macOS, and in the future, I would love to see first-class support for BSDs and Windows, platforms that can definitely be supported and have been to some degree already.

I do think that neither Nix nor Guix will fully replace the ecosystem of package managers on Linux, for a variety of reasons, but I think Nix is going to be one of those tools alongside Flatpak and Brew that users use alongside native package management on their platforms. Nix and Guix have some very unique advantages to go along with their disadvantages.