Earlier quoted context omitted.
There is no gatekeeping. To actually achieve POSIX-compliance is not an easy task and requires lots of testing. If you don't do it then your replacement will break everyone's scripts. A distro maintainer will also want to retain compatibility with their supported GNU/BSD extensions so that's more work to add on. I would still agree with his statement at least as far as BSD is concerned. Outside of Redox I haven't see…
To replace anything in OpenBSD, you need way more than POSIX-compliance -- you need OpenBSD-compliance. Which means every non-POSIX option has to be implemented down to edge cases, every departure from or controversial interpretation of POSIX (POSIX is vague in many places) has to behave exactly the same. Or you'll definitely break scripts.
.ls -1 /bin/ /sbin/ /usr/bin/ /usr/sbin/ | sort -u | wc -l
671
Good luck reimplementing that.