So "cooperative multitasking is not preemptive multitasking".
The typical use of the word "asynchronous" means that the _language is single-threaded_ with cooperative multitasking (yield points) and event based, and external computations may run concurrently, instead of blocking, and will report result(s) as events.
There is no point in having asynchrony in a multithreaded or concurrent execution model, you can use blocking I/O and still have progress in the program while that one execution thread is blocked.
Then you don't need the yield points to be explicit.
While this is indeed the most common use, I'll bring as counter-examples Rust (or C#, or F#, or OCaml 5+) that supports both OS threads and async. OS threads are good for CPU-bound tasks, async for IO-bound tasks.
The main benefit of having async (or Go-style M:N scheduling) is that you can afford to launch as many tasks/fibers/goroutines/... as you want, as long as you have RAM. If you're using OS threads, you need to pool them responsively to avoid choking your CPU with context-switches, running out of OS threads, running out of RAM, etc. – hardly impossible, but if you're doing more than just I/O, you can run into interesting deadlocks.
> The main benefit of having async (or Go-style M:N scheduling) is that you can afford to launch as many tasks/fibers/goroutines/... as you want
Some have argued that the real solution to this problem is to "just" fix OS threads. Rumor has it Google has done exactly this, but keeps it close to their chest:
Somewhat related and also by Google is WebAssembly Promise Integration, which converts blocking code into non-blocking code without requiring language support:
I think that PI will have a role, but I suspect that it can quickly destroy the performance of wasm code in unexpected ways.
As for fixing OS threads, indeed, this may very well change the ecosystem, but many developers expect their code to be cross-platform, so it might take a while before there is a solution that works everywhere.
The typical use of the word "asynchronous" means that the _language is single-threaded_ with cooperative multitasking (yield points) and event based, and external computations may run concurrently, instead of blocking, and will report result(s) as events.
There is no point in having asynchrony in a multithreaded or concurrent execution model, you can use blocking I/O and still have progress in the program while that one execution thread is blocked. Then you don't need the yield points to be explicit.