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.
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.