There are probably still ways to maintain a source archive with a ports system. Just analyze the sources used by builds, create a mirror, and redirect fetches to use the mirror. It's not that crazy. The packaging would still be a separate affair.
This is correct; in Nix lingo these are referred to as "fixed output derivations". For some other interesting context, see this Nix forum post from last year in which they discussed deleting some stuff from cache.nixos.org to save money, but were clear that they'd keep all fixed output derivations and only delete other things that aren't derivable from those: https://discourse.nixos.org/t/upcoming-garbage-collection-fo...
