Well, you have two tools that can fetch stuff - git and cargo. In my preferred workflow there'd be only one tool. It would also work as a cloud filesystem, like p4fuse.

Cargo can use git, but it doesn't primarily. Crates are stored as tarballs on S3.

Right, I don't see why that's necessary. The whole world could be one big source control repository where each client keeps only their local changes. So e.g. if you want to make a local tweak to a third party library you're using, you 1) edit their file in your filesystem which is a transparent view into the world repo, 2) recompile your project.

That forces everyone to use git, which is not something we're interested in doing.

Agreed, it would force everyone to use the same VCS. I've lived in this kind of setup for years and I'm very happy with it, but point taken.