Do you not view io_uring or Windows IOCP as alternatives? Or the plethora of other non-POSIX IO extensions in Linux, such as splice and sendfile? SPDK and DPDK? There’s also Google’s Fuschia operating system, which is not Posix. I think it’s pretty clear that people are investing in a variety of alternative approaches.
To look at things from another angle, what if having had POSIX all these years actually made things more uniform, in the sense that we didn't have variance between systems as large as e.g. between a BSD Unix and a Lisp machine? What if that were our choices today. As it is the largest mainstream departure is maybe Windows.
I don't simply due to the fact that no one ever bothered to put something together on UNIX-like systems that resembles any of those options.
If anyone felt that those were worthy ventures, in these last few years we would already have seen something resembling that effort. But we haven't.
What do you mean? io_uring is a Linux kernel interface, and there are several userspace projects that are building upon it.