Some of Rust's async runtimes seem pretty similar to "optional green threads". There's no garbage collection; you can get something like a reference-counted GC by using Arc everywhere, but it's neither automatic nor universal.

Would we have gotten to "optional"? Maybe! But it's hard to predict the counterfactual, especially when substantial success usually has components of both design and luck/right-place-right-time. Rust hit a sweet spot, and it's not clear how a different history of Rust would have turned out.