It may not really help the case, but I firmly believe that it is not the language, but the ecosystem, and is more of a fundamental issue. But maybe putting the blame elsewhere could help accept the situation.

So anyways, the language is pretty much a lazily evaluated JSON. But even if it were something else (insert your favourite language), the problem ultimately is that packaging software is complex especially in a non-standard way, with endless edge cases, requires whole libraries and conventions and this is simply not a well-trodden path. Most programs simply hard-code "traditional" Linux file system conventions and those have to be patched in some way.

So the hard thing is not "is this really a function application here", when writing new Nix code the hard thing is simply knowing that for python there already exist this abstraction in nixpkgs, but to use it you need this folder structure and this build tool, etc. Especially when there are multiple abstractions for the same thing because it's an absolutely huge repository with countless packages.

But the benefits absolutely make up for it big time - there is simply no going back from Nix imo. I would honestly feel constantly "dirty" with any other traditional package manager, it's like file "versioning" before version control.

(PS: just grep for use cases of a function you are looking for. Also, find a "blueprint" package and start from there, e.g. another program written in python with a few native deps)