I don’t like this because it assumes that all of those repositories are accessible all of the time to everyone who might want to build something. If one repo for some core artifact becomes unreachable, everyone is dead in the water.

Ideally “cached on the network” could be a sort of optional side effect, like with Nix, but you can still reproducibly build from source. That said, I can’t recommend Nix, not for philosophical reasons, but for lots of implementation details.