This points at the bigger problem that every implementation of green threads has: it breaks interop unless everybody buys into it. Async/await is fundamentally callback-based, which can be represented nicely even in C ABI - so literally any language that can interop with C, can interop with an async/await model, with execution flow asynchronous throughout the entire chain. Runtimes that choose to roll their own green threads, like Go, and I guess now also Java, are effectively stating that their convenience is worth the inability to interface well with async code outside of their runtime.
> Runtimes that choose to roll their own green threads, like Go, and I guess now also Java, are effectively stating that their convenience is worth the inability to interface well with async code outside of their runtime.
You'd still be able to interop with async code outside of runtime, you can still model the API as callbacks for the interop purposes where needed.
But yes, interop has sam trade-offs. Which are, IMO, entirely correct trade-offs to make for Go, Java and .NET (and overwhelmingly so). But they are probably not for (as an example) Rust, according to it's goals.
As for C#, some kind of fibers appear to have been considered but deemed too hard or not worth the trade-offs. See here: https://github.com/dotnet/runtime/issues/11084
The acceptable alternative was, apparently, forcing to programmers to litter their code with words such as async/await/task in-between the words and symbols that actually describe the purpose of their program, should they want to take advantage of non-blocking IO. All the way throughout the call-stack, nonetheless. And you also get crap like this: https://github.com/dotnet/corefx/pull/4868