Seeing Repology stat on how many projects are in nixpkgs, how fresh they are, you are very largely wrong... Snaps/Flatpack have very few and mostly outdated packages. They are behind ANY other package system. NixOS is the largest set of packages and the most fresh. Arch and Guix follow short. The others are behind.

Not only, try to see how many "default.nix" or "shell.nix" you'll find in FLOSS projects, like in the past the "debian" dir. and you'll surely have an Idea.