Comment by throwawaymaths

I am not on the core team but i believe the plan is to do exactly what you are talking about, but after the API is nailed down and kinks have been ironed out by users of the existing semiblocking implementation (to possibly include the compiler), as the default LLVM coro state machine compiler has problems (for example: I think I remember Andrew complaining that it has an obligatory libc/malloc dependency).

since the new io interface has userland async/await methods, then dropping in a proper frame jumping solution will be less painful, and easier to debug, and if using coroutines proves to be challenging with the api hopefully changes to io api would be minor, versus going after stackless coroutines NOW and making large API changes often as the warts with the system uncover themselves.