You don't need kernel scheduling for stackful threading. So-called green threading was common long before event-oriented asynchronous programming became popular with the advent of kqueue and epoll. See, e.g., https://www.gnu.org/software/pth/

That said, you can make kernel-aware user-controlled context switching much faster than it is currently: https://www.theregister.com/2020/08/10/google_scheduling_cod...