It's pretty clear that most people don't want to write and maintain that kind of code, though—that's what async/await is for. Personally, I hate writing state machines.
Async is deinitely a distraction. Rust should have stabilized other pieces more before jumping to yet another paradigm. It is gaining this collection of half-implemented things. I'm guessing generators are going to the the next move before async/await is even in the print book.
Rust has moved from zero-cost abstraction to "an fairly efficient implemention of everything you can think of".
You can see it here on HN. What's the #1 comment that always appears on Rust async threads? Invariably it's "why not just use goroutines? That kind of code is so much easier to write!" Now for various reasons M:N threading like in Go doesn't work in Rust. What does work, and offers most of the ergonomics of threads/goroutines, is async/await. That's why the team has prioritized it.
There are people who care less about ergonomics than optimizing every CPU cycle for socket servers. For those people, the focus on getting the ergonomics right before microoptimization of mio and tokio might be frustrating to see. But the overwhelming majority of users just want easy-to-use async I/O. Async/await is an enormous improvement in readability and maintainability over hand-written state machines. Rust would be doing a disservice to its users if it focused on microoptimization of tokio instead.
Not sure exactly what you mean here; mio predates tokio by a couple years, and has been possible to use standalone since before tokio ever became a thing.