The problem is tokio only insofar as it blocks attempts to develop an unified, common denominator API between multiple runtimes (for example: how can the Rust ecosystem not yet have a standard async reader trait, after years and years?), and instead encourage all sorts of libraries to depend on tokio directly rather than a facade that works on multiple executors.

Right now cross-runtime libraries are mostly written special-cased: one feature flag for tokio, another for async-std, maybe one for smol if they feel fancy. Almost none for glommio or other runtimes. That introduces a huge burden for libraries, that would rather depend on a single API.

Rust shouldn't have an official runtime; it should have APIs that make possible to write libraries that don't dictate which runtime you must pick.