Unix was not exactly Multics's successor. It remains unclear if the Unix team even had the rights to rewrite "something like Multics" when they started.

The fact that ksh was "opened" quite late did not stop its success as its language has been widely adapted by then. Concerning root shells, the csh (or TENEX csh) is probably even less common, FreeBSD uses the Almquist shell (like NetBSD does AFAIR).

Probably related: ksh adaption list.
http://www.in-ulm.de/~mascheck/various/s...sions.html