What we're doing in Java is letting you choose, for each sequential computation, whether you want a heavyweight (kernel) thread or a lightweight usermode thread

What we're doing in Java is letting you choose, for each sequential computation, whether you want a heavyweight (kernel) thread or a lightweight usermode thread (like a goroutine), and if you choose the latter, you can use your own scheduler (schedulers are written in Java, and aren't a part of the runtime). No promises, no observables, no async/await, and no thread control issues.