...
> POSIX isn't necessary to the HFS.

That is basically correct in that we could have implemented a more
generic hierarchical file system/directory structure without
complying to the IEEE P1003.1 standards which begin to define
"POSIX".

MPE was changed internally to support some of the POSIX functions,
with HFS and long files names being some of the changes.  However,
more often, we chose to implement a POSIX feature using existing
MPE data structures and basic elements. And, there are several
POSIX features that we did not implement due to high cost/risk,
or due to breaking the MPE model. E.g. a process surviving the
death of its parent, a file having multiple physical (hard) links,
multiple effective GIDs, the GTI specs, select() for terminals, etc.

As Gavin (I think) mentioned, you can always delete the group
HPBIN.SYS and the C runtime libraries /lib/libc*. I do NOT recommend
doing this! Less drastically, you could add a lockword or create a
restrictive ACD to SH.HPBIN.SYS

I agree that POSIX on a Unix OS will be always be more compliant than
POSIX on MPE. I also agree that yanking the pieces of POSIX out of MPE
would be a huge waste of time and could de-stabilize the existing code.

FWIW,
 Jeff Vance, "CSY"