Since I can't edit the above comment, one additional thought is that your criticism is valid for a language like Python, which has a much higher tolerance for abstractions such as green threads being built into the language itself. It would've been interesting if Python had gone with a solution other than async/await.
I'm also a little confused when you say "Rust started with green threading/fibers". I think the term "fiber" is overloaded here, but Rust did start with green threads (M:N preemptive multitasking). Rust now has support for cooperative multitasking via async/await. From what I can find of UCMG, it kind of misuses the term fiber. Looks like it's just green threads that the kernel is aware of?
I'm also a little confused when you say "Rust started with green threading/fibers". I think the term "fiber" is overloaded here, but Rust did start with green threads (M:N preemptive multitasking). Rust now has support for cooperative multitasking via async/await. From what I can find of UCMG, it kind of misuses the term fiber. Looks like it's just green threads that the kernel is aware of?