I would like to add a feature request to the issue tracker for `guix package -u` to have an option for downloading only binaries. If the server hosting the binaries is down then it doesn't build locally from source. I don't know how involved this might be given how guix is designed. My use case is that I have a slow laptop, and it's inconvenient to build packages from source. This would be like the --no-substitutes option, but it would be the opposite, --no-build.