[pxc]

That's actually really important. Sometimes upstream sources do disappear, and the caches we have don't help very much, or only help for the exact collection used in a specific package, when they're one big chunk like with buildGoModule.

I hope that FODs in Nix itself stay flexible, though, even if a more limited form becomes preferable in Nixpkgs. New fetchers being possible to implement in Nixpkgs rather than in Nix itself seems really valuable, and clunky uses of FODs to wrap upstream vendorization are still an important fallback option for wrapping new or ill-behaved upstream package managers, until better implementations are worked out or become possible.