If Rust had gone with green threads as the core async strategy (I know it was a thing pre-1.0), that would be terrible. You're not understanding Rust's design goals. Rust's async model, while it has several major pain points at present, is still undoubtedly superior for what Rust was made for. It would be a shame to throw all that away. Go can go do it's own thing (it has, evidently).
Rust: it turns out that not every concurrency needs to be zero-cost abstraction