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...