Not having a required input, say when you try to reproduce a previous build of a package, is a separate issue to an input silently changing when you go to rebuild it. No build system can ensure a link stays up, only that what's fetched hasn't changed. The latter is what the hash in nix is for. If it tries to fetch a file from a link and the hash doesn't match, the build fails.

Flakes, then, run in a pure evaluation mode, meaning you don't have access to stuff like the system triple, the current time, or env vars and all fetching functions require a hash.