It would be better to have blessed crates in crates.io. The Rust core team would release or audit them. If the blessed crates need breaking changes, it can be done by increasing their major semantic version number. That can't be done to the standard library.
Actually, there could be a "trust" level for crates: 1. blessed crates by the Rust core team, 2. trusted developers, 3. untrusted developers. Or something like that..