I really hope to see a lot of packages move out of nixpkgs, only to be re-imported as flakes. There are currently 3.9k pull requests for nixpkgs; the current model really isn't working.
I don't know if packages should necessarily be moved out of nixpkgs since I doubt most people who don't use nix would want to maintain flake.nix in their project, nor would users want to have 30+ flakes in their inputs. However, I can see a good case for separating the package definitions from the nixos modules and the stdlib (mkDerivation, stdenv, etc). Maybe there could be a flake for each.
As for keeping up with PRs and other responsibilities, that seems to be one of the stated goals of the this new Nix Team. I guess we'll see how they intend to handle this soon.
Look at the ratio instead: 4k open vs 162k closed. Or 4k open vs 80k+ packages.
If we split things up, there will be less open PRs per project of course. But I doubt the ratio will be much better. It will actually increase the number of PRs as large scale refactors will be multiplied by the number of repositories.
As for keeping up with PRs and other responsibilities, that seems to be one of the stated goals of the this new Nix Team. I guess we'll see how they intend to handle this soon.