I don't think it's a good idea for Rust to choose a particular kind of concurrency primitive. Rust is pretty broad in its use cases, and choosing a primitive may end up restricting other fields. Remember that Rust used to have a libuv based green threading model, and it was removed for precisely this reason -- it affected some use cases in major ways.

Rather, it is important for subcommunities to rally around particular sets of libraries. E.g. the web and I/O related libraries seem to all be going in the direction of integrating with tokio. This means a web programmer can use tokio and it will work smoothly. OTOH, a different subcommunity that uses a different set of libs may use stuff built on rayon and crossbeam.