Semantic versioning is culturally widespread in Rust, so the problem of library authors being "less careful about API stability" rarely happens in practice. If pinned packages were the problem, I'd imagine they would have been called out as such in the Debian page linked by parent.

Semantic versioning is a way to communicate how much a new version breaks new shit not a way to encourage not breaking shit. If anything, having a standardized way to communicate that you are breaking shit kind of implies that you are already planning to break shit often enough for that to make sense.