I'm pretty sure NixOS isn't the only one doing this hack. The Yocto build system does something similar. It contains its own build-time binary glibc, and patches its tools to point to its own internal library installation. Or something like that. In effect, Yocto has its own build-time distro, which has to run from any filesystem location.
> I'm pretty sure NixOS isn't the only one doing this hack
When developing ArchMac I had to do godawful hacks to bog-standard libs because whatever build system decided to hardcode a lib path (or forcefully strip one when it should be hardcoded, I've had to handle both) that I had to manipulate through various means including install_name_tool which is not that different from patchelf†.
This kind of issue was not macOS specific, it just turns out the various ways things were built happened to gracefully "work" on most Linux distros by sheer luck but they could have been equally broken.
† Not really a surprise when thinking about it, the concept of Nix derivations is not that different from the concept of Darwin bundles/frameworks (in terms of being a self-contained dependency package) so it's only natural similar issues, and thus approaches and tools to tackle them, emerged.
Scripts and environment variables are an ugly hack. Environment variables are dynamically scoped and will cause problems, if you don't care to suppress them from being inherited by child processes.
Suppose there are two installations of app. One is the system one, and one is locally installed by the user. The user overrides LD_LIBRARY_PATH when invoking the local app. Suppose that that app is used in such a way that it invokes the system-installed app; that could then find the wrong libraries due to the LD_LIBRARY_PATH being inherited.
A program must simply know where its exact pieces are, all by itself, without any external tricks that could influence more than just that program.
Search paths (all of them, including PATH) should be left to the user, for arranging the system; the user should be able to manipulate paths in arbitrary ways, yet the application shouldn't break as far as being able to locate and load its own pieces.
