> that must in turn be backed by a multithreaded event loop to be useful

Why? You can just as well execute all your coroutines on a single thread. Many networking applications are doing fine with just use a single ASIO thread.

Another example: you could write game behavior in C++ coroutines and schedule them on the thread that handles the game logic. If you want to wait for N seconds inside the coroutine, just yield it as a number. When the scheduler resumes a coroutine, it receives the delta time and then reschedules the coroutine accordingly. This is also a common technique in music programming languages to implement musical sequencing (e.g. SuperCollider)