It depends on how feasible stable Rust is for popular projects. Many projects mandate nightly. Even if it is a fixed nightly version, the discrepancies across nightly versions add up since they don't just add APIs, they remove them too.
Unstable rustc can compile stable code. For code you'd write with Dafny I highly doubt it would be an issue. Perhaps you have a proof of the opposite?
I've only ever needed nightly when doing something a bit off the beaten path, like targeting microcontrollers. The Linux kernel is another big project that needs unstable. It's much more rare a need than you make it out to be.
