The key difference is cooperative multitasking lets the program yield the thread anywhere, not just to the event loop like async programming. Arbitrary yielding was a feature that programmers widely abused in the early Windows days. The user would start something in an app that takes some time to complete; the app would freeze for a while, but all other apps remained usable. It was obvious that the programmers, rather than solving the real problem, had sprinkled some yield instructions throughout the program, which allowed the computer to keep working even though the app was unresponsive. It's a good thing that async programming frameworks don't usually allow yielding from arbitrary places.
True. That's the new kind of yield that requires language support and it's only available in async functions. I was referring to what happens when framework or language designers try to allow something like the await keyword in non-async functions; it turns into an epic mess. I know because I tried (as a thought experiment.) :-)
