For a lot of use cases, there really isn’t any important difference between 100% and 0.1%. If any of the source tarballs are unavailable, then I can’t produce a build, and the fact that some of them are available isn’t much consolation. I’m blocked either way.

Especially for an organization like Arch Linux, the solution is really obvious; maintain your own mirror. Debian and Fedora do it, so can they.