Why does a language need higher developer velocity? Why is acceleration needed?

Acceleration makes sense for end user applications, where you're allowed to break whatever you see fit, but I don't see how it helps with core infrastructure like a programming language, where every mistake is multiplied a million fold.

Exactly this. There's plenty of boring software out there where writing the actual code is a chore. That's where AI works pretty well, as long as the people at the controls understand and can explain the problem and non-functionals.

For projects like Rust, coding velocity is not the bottleneck, speed / productivity is not a priority.