Either the split is "every use-case finds the primitive that works best for it" (Rust), or the split is "here's the blessed primitive, other use-cases can pound sand" (Go)

For example, while tokio is being integrated with hyper, it does not affect the "usual" users of hyper at all, both in terms of API and in terms of what's going on under the hood, cost-wise.

Having a blessed solution for async like tokio that is not in the stdlib seems to encourage folks to integrate with it, but not in an irrevocable manner; the core library stays intact and able to support potential other models in the future.