> if you own it and know its structure

This is never the case for a async/await generated future.

> Why would you ever want to wait on a Waker that you won't wake? :)

Blindly waking a Waker is still the same as busy polling. You need to know whether what the async fn requested is ready or not.

> It's not more difficult than with the manual state machine?

For the end user? No. For whoever is writing the state machine? Yes. But those are generally different users.