If the same project had been packaged with Nix, it would probably still compile. People regularly checkout older versions of nixpkgs to get access to older pack

If the same project had been packaged with Nix, it would probably still compile. People regularly checkout older versions of nixpkgs to get access to older package releases. One of the key property is that the build system enforces all the build inputs to be declared. And the other one is to keep a cache of all the build inputs like sources because upstream repositories tend to disappear over time.