You can have UI workflows just as well with a language that has (runtime-managed) green threads, like Go and Elixir. It's just that you sometimes have to make sure that certain green threads have their actions scheduled on the UI (OS) thread for backwards compatibility reasons. For example, the Scala ZIO library provides the means to control where (on which OS thread) code is scheduled in a very intuitive way that does not involve ConfigureAwait hacks. ZIO is not green threads and rather more like async/await, but a similar approach could be implemented in languages with runtime-managed green threads as well.