That's a very good point. However, choosing to use a specific version already implies forgoing anything newer than the next release. This would include language features and crates (generally).
Thankfully, crates.io lists the dates next to crates so that users can select a version that is released before or during the release cycle of their compiler, thus guaranteeing compatibility.
If you're willing to stick with an older version of cargo and rustc, why not?