In my mind, it's that updating a dependency doesn't break existing installations, or knowing that an existing install isn't going to get borked by an update.
And this is not something that is applicable to ecosystems like Rust, where it's not really possible to break a Rust program because another Rust program needs a newer version of the same dependency that happens to be incompatible with the older version. In fact, you can compile one Rust program that links against multiple versions of the same dependency at incompatible versions without issue.
So the entire notion of the Debian model of package management doesn't really apply to Rust, and there's not any benefit to keeping an older version of the toolchain around. There are only negatives.
And Rust has strong stability guarantees. A newer toolchain will not break compiles of older code. Nor will Cargo's cache break compiles with an existing lockfile because another package needed different incompatible versions of the same dependency. It's designed to be stable from first principles, in a way that C and C++ builds are not.
This is kind of why you're only going to have a bad time if you want to use the system package manager to manage your Rust packages. It's not built for the same problem domain, and over constrained for the design space.
