I do wonder how nix could solve the nixpkgs problem. In order to build it actually requires the recipes to calculate the hash with which it builds the store path.

Fixed output hashes could be part of the solution. A map of package and version to hash and outPath maybe? But for building from scratch e.g new packages, where would the recipes be stored? I think it would have to be in git, but where would that be stored? A monorepo again? Then we’d be back at nixpkgs.