It’s probably very painful to try to convert a piece of async software to use a threaded model (though I’m just guessing, having never had to attempt this), so

It’s probably very painful to try to convert a piece of async software to use a threaded model (though I’m just guessing, having never had to attempt this), so I’d say it’s more a matter of thinking up front about whether you need to control threads directly. But to your main point, I agree. I suspect the vast majority of software has no need for thread-model parallelism, and languages have only historically presented that parallelism model because it’s the model that the OS exposes to userland, even though the OS scheduler is async-style scheduling the machine threads themselves!