Nix living at a predefined path is integral to how it works. An executable does not dynamically link to a generic "ncurses" but (via rpath) links to a specific compiled version of ncurses (such as /nix/store/81rb87agmp9cbsvg2xm2n4kp9c6309lv-ncurses-6.2). This is the root of all the benefits of Nix such as being able to install things side-by-side that use different versions of things or upgrade and rollback without problems.

That predefined path being the same (/nix) across all users of nixpkgs is required to be able to share binary packages (you could perhaps build everything from source, but that's a lot of time, more time even than something like gentoo because package updates require all dependencies to be rebuilt as well).

You can call it an insane choice or bad design, but there aren't a whole lot of options here. Could Nix move to a different path? Maybe, but is there a path that all operating systems could abide? If the new path stops working in some future OS, will it still be insane and bad design? Again, maybe, but I happen to love Nix and I use is on macos because it makes my life easier (and I'm on macos for work reasons). I'm willing to bend and do a lot of legwork to be able use Nix, and I'm upset with the Catalina situation.

Can follow some discussion here https://github.com/NixOS/nix/issues/2925