Those layers would have to be reimplemented to retain compatibility.
> I think you're hugely overstating the importance of POSIX.
People generally understate importance of POSIX, just because it's old. It's impossible to get APIs perfect and you are throwing away decades of work in the name of getting APIs "right". No one is going to rewrite everything from scratch, just because the new APIs look shiny. For reference read about Unix wars.