> Having a repository with a large number of packages doesn't prevent you from writing Nix code outside of that repository.
No, but it does define how (and where) I will write that code.
It's not uncommon for a package to need complex dependencies like udev. It's definitely necessary for nixpkgs to have opinionated decisions about how dependencies are structured, and what options/defaults are available and where. The problem isn't the quality of nixpkgs, or with any of its design decisions. The problem is that those design decisions are centralized and interdependent. That's the core issue that lead nixpkgs to be so poorly documented. It's the core issue that makes it incredibly challenging to create a sensible UI/UX for Nix and NixOS.
There are definitely benefits to a monolithic source tree. You can guarantee compatibility. You can have a clear place for communication about breaking changes and security updates. The thing is: you can get all of this without a monolithic source tree. Debian has been doing it for 30 years now. If anything, Nix should be able to do it better!
If the goal is to have a diverse set of package repositories, what better way to deliver them than a purely-functional declarative package manager? Nix is the one package manager that can handle totally incompatible packages, breaking changes, etc. all without breaking a sweat; yet at the same time keeps all its packages in a single monolithic source tree where none of that is allowed to be present. I see it as a missed opportunity.