But you wrote "Rust has no backward compatibility" who cares what libraries/projects do?
Rusrc nightlies are the compiler. The only notable compiler i am aware of.
There is no standard, ecosystem or any -std=v1.0 that old code can rely on.
>who cares
People who want to invest time in Rust.
Servo might use nightly, but do you have evidence that most projects do? Especially after custom derive landed in stable, most of the reason for sticking with nightly dissappeared.