The decision to put the /nix directory in the root rather than in /usr or /usr/local seems a deliberate provocation to any self-respecting distro. Since many binaries contain hard-coded paths, this isn't a decision that a distro can easily correct either.
Nix package manager should not break if you change the location of the nix store. However, you will end up having to recompile everything from scratch.
As far as I understand, hardcoding paths and /nix are both deliberate decisions to prevent unexpected implicit dependencies. It does seem like the cheapest way to do it.
I don’t understand why distros would be so attached to FHS? I don’t think there’s ever been significant amounts of consistency between different distros and other Unix OSes when it comes to the details. I’m fairly sure they could build a FHS layer on top if they really wanted to, since the paths inside packages in Nix are fairly standard.
