Now you are comparing single threaded code with multi threaded, which is a completely different axis to async vs sync. Just take a look at C#'s async, where you have both async and multi threading, with all the possible combinations of concurrency bugs you can imagine.
Of course I'm comparing them. Threading and async are two solutions to the same problem: How do you write high performance event driven systems like network services? How do you solve the C10K problem (or more recently the C10M problem)?
If you use a thread per connection (or green threads like Go), you don't also need async. If you have async (eg nodejs), you can get great performance without threads. You're right that they can also be combined - either within a single process (like tokio in rust). Or via multi-process configurations (eg one nodejs instance per core, all behind nginx). But they don't need to be. Go (green threads) and Nodejs (async, single threads) both work well.
Of course we're comparing them. We all want to know who wore it better
You wrote:
> The job wouldn’t have been done. They would have needed threads. And mutexes. And spin locks. And atomics. And semaphores. And message queues. And - in my opinion - the result would have been a much worse language.
My point is that you do need mutexes, spin locks, etc with async as well, given that you have a multi threaded platform. So no, we have basically 2x2 stuff we are talking about with very different properties (async/sync x single/multi threaded).