posix isn't a kernel API. It is a runtime that can largely be implemented in userspace. There is no need for any kernel to do anything specific to support posix

posix isn't a kernel API. It is a runtime that can largely be implemented in userspace. There is no need for any kernel to do anything specific to support posix other than for possible performance optimizations. Also posix isn't even the API people target anymore - the Linux uapi is. This is why wsl and starnix target that instead. There are security issues that come along with such APIs, but you can sandbox things in such a way that it need not be as problematic. There is no getting away from the need for backwards compatibility with existing applications. Applications won't start supporting your platform natively until it matters and getting big enough to matter requires lots of software working on your platform.