Where do you think the Io parameter comes from? If you change some function to do something async and now suddenly you require an Io instance. I don't see the difference between having to modify the call tree to be async vs modifying the call tree to pass in an Io token.

Synchronous Io also uses the Io instance now. The coloring is no longer "is it async?" it's "does it perform Io"?

This allows library authors to write their code in a manner that's agnostic to the Io runtime the user chooses, synchronous, threaded, evented with stackful coroutines, evented with stackless coroutines.