> And since async/await is more conceptually equivalent to a state machine than a coroutine or sync/blocking code, that severely limits how well we can reason about large promise chains.

I have to disagree with that. You can implement async/await using state machines, just as you can implement a loop using goto, but it's a more constrained model that's much easier to reason about than a fully general state machine or a fully general coroutine. The control flow still proceeds the way you expect - you still execute code from top to bottom, you still return once from every function call - you just yield at some points in it. You're not interleaving your control flow with some other function that you communicate with (like a generator), and you're certainly not treating suspended control flow as something to be copied or passed around (like a coroutine). It's a much simpler concept.