Debian packages integrate better with the system, are authenticated by gpg, and have at least one more critical pair of eyes on them.

If you're willing to stick with an older version of cargo and rustc, why not?

Because the ecosystem is not. A lot of what’s on crates.io needs the latest and greatest version.

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.