You get that anyways. The question is how popular something like tokio will get. Since its blessed by the core team and the language designers, I think it will get popular, and therefore many will use it. But there will inevitably be someone who doesn't like it, or for whom it doesn't fit, and they will use something else. But that happens no matter if tokio is in the stdlib or not.

That's obviously a threat. But so far this is not what's happening: since tokio and future-rs were announced we've seen most library authors ditching their old stuff to use tokio instead.

Not really. People in the Rust community have shown a great deal of willingness to standardize on a handful of traits, so things tend to be intercompatible. That might change, I guess. In any case, there's no point in using an async I/O model for the majority of use cases many of the people using Rust are interested in, so people standardizing on something else for those usecases isn't a problem.