It would make sense to have an official default async runtime in the standard library while keeping the door open to use any other runtime, just like we already have for the heap allocator or reference counting garbage collection.

There are issues in particular with core traits for IO or Stream being defined in third-party libraries like tokio, futures or its variants. I've seen many cases where libraries have to reexport such types, but they are pinned to the version they have, so you can end up with multiple versions of basic async types in the same codebase that have the same name and are incompatible.