Do you really expect any project to have nightly builds that are forward compatible? The whole point of regular development builds is to have a place to test and do experiments... which is pointless if every experiment needs to be supported forever.
The "namespace hack" in Rust is having the "stable" namespace: a compiler that rejects all unstable code (and is forward compatible), so that people don't accidentally get pinned.
If someone wants to pin to a specific development version (of anything), there's nothing the project can do about that, other than being closed source and only publishing stable artifacts. Tooling like rustup that makes it easier to pin means that people who choose to experiment can still use stable by default (easy to switch back) and that others can easily collaborate on the experiments, both of which seem like strict positives.
The "namespace hack" in Rust is having the "stable" namespace: a compiler that rejects all unstable code (and is forward compatible), so that people don't accidentally get pinned.
If someone wants to pin to a specific development version (of anything), there's nothing the project can do about that, other than being closed source and only publishing stable artifacts. Tooling like rustup that makes it easier to pin means that people who choose to experiment can still use stable by default (easy to switch back) and that others can easily collaborate on the experiments, both of which seem like strict positives.