> Since there is a single GUI thread where everything that touches the GUI must live, but you do want your application to have multiple other threads, you need to give the programmer this kind of control and can't rely on a more simplistic Go-style green threads runtime.

Isn't that just a limitation of Go since its goroutines all run in the same pool? As far as I know, in the Java implementation you'll be able to create virtual thread pools from regular threads (i.e. the gui thread) and then have another virtual executor for the rest of your program. You'll still have the backend vs gui split but you won't have the entire async await split for the rest of your application.