This is not quite true. There are some assurances around availability that crates.io provides that self-hosting wouldn't. It won't publish two crates with the same version, it won't completely delete a crate version (a yanked crate-version is still available if you have a Cargo.lock file), it won't retroactively change versions, and it won't completely go away if a domain expires. URLs in their ideal are great. In practice they are brittle.