I mean, Nix is pretty complex software, and is an added layer of abstraction in many contexts. Framing Nix as a solution to complexity seems to be a tenuous claim.
What Nix can help with, imo, is reducing toil. And a good abstraction maintained by a team can reduce toil for a lot of others.
Disagree strongly. Building a large and varied set of software packages in a generalised and reproducible way is a fundamentally complex endeavour, and nixpkgs is the most successful effort I've encountered to simplify it.
In contrast, build & packaging systems that try to present apparent "simplicity" are usually ignoring a lot of the subtle difficulties in building software in a reproducible and robust manner, leading to extended and vague debugging sessions. See for instance almost any non-trivial Dockerfile-based build procedure.