Comment by Gladdyu

If you would write a basic scheduler, at some point you'd have to await the userspace code but you wouldn't have any way to force it to stop running. If the userspace code would enter an infinite loop it would hold the kernel thread forever. Within a constrained environment, eg. the kernel itself (and even that's sufficiently complex with loadable drivers that you might end up with bad interactions) I could see some use for async await, but you'd still need to be able to preempt untrusted code.