All "work" you want to do that interfaces with anything on an OS is handed off to the kernel; want to read a file? kernel, want to sleep for a while? kernel, etc. Besides things like network traffic is also asynchronous like io_uring (even if the socket() interfaces make it look somewhat synchronous). Outside of toy system asynchronicity is always a thing, especially when running on multiple cores.

I kind of get where you are coming from but at the same time, the kernel always gets the last say, so as long as io_uring has a good design and implementation it will always be just as good or bad as the OS as a whole. Whether run of the mill programmers are up to the task of being able to properly conceptualise and use such an OS is probably not the same thing.