If you need to be explicit about thread contexts because you're using a thread that's bound to some other runtime (say, a GL Context) or you simply want to use a single thread for synchronization like is common in UI programming with a Main/UI Thread, async/await does quite well. The async/await sugar ends up being a better devx than thread locking and implicit threading just doesn't cut it.

In Java they're working on a structured concurrency library to bridge this gap, but IMO, it'll end up looking like async/await with all its ups and downs but with less sugar.

"Green Threads" as implemented in Java is a solution that solves only a single problem - blocking/multiplexing.

It does not enable easy concurrency and task/future composition the way C#/JS/Rust do, which offer strictly better and more comprehensive model.