In the same imaginary world, creating a new POSIX OS that isn't Linux is a giant waste of time just from gap in drivers alone. If you're going to ditch all that hardware support then you may as well ditch POSIX and its crap while you're at it.
Bull. People developed software for multiple completely different computer architectures throughout the 80s and 90s. People do it today between different game console platforms in addition to operating systems. Even if you try and target something like SDL or a web browser your abstraction won't save you from a platform's quirks once you reach a certain level of complexity and then you'll have to work around it anyway.
Hell, even between supposedly POSIX systems there's a lot of #ifdef going on to make things work.
My issues with POSIX stem from the fact that writing completely correct code which handles signals, interruptable operating system calls, and threads is hard. There are plenty of little details that are easy to get wrong. And you won't know you've gotten something wrong until much later when some confluence of events occurs.
I don't know if deprecating parts of POSIX is going to work any better than deprecating parts of C++. If all the bad stuff is still there waiting to be misused...
Or successfully pull off the library OS concept with POSIX as a first class citizen. Z/OS is most of the way there. NT has tried a couple times with the POSIX subsystem first, and now their Windows Subsystem for Linux work came out of MS Research's drawbridge library OS work.
As long as you don't have POSIX only special interest users will use your system.
I believe the only way forward is to start on POSIX and then move on step by step and deprecating part by part.