Early in my career, one of my first professional jobs was in a hard core korn shell/C shop. Easy stuff was done in korn. Anything requiring more than a page we wrote in C. Now, as I read this post, I think you're trying to focus on negative reasons to not use shell. Some of them seem forced to me (slow? What ksh programmer worth their salt uses, oh, an external call to sed or awk for what a read loop would do?).
in reply to Unix shell versus Perl
I think it misses the point entirely. The main reasons to switch are power and productivity. Rather than beating people out of skills they might be expert at, show them how much more can be done with a reasonably sized Perl script. If you tell them they'll get the same jobs done in a fraction of the time, a fraction of the space, and the result will do more than they could with shell, then you have a potent argument.
Tell them they can write daemons in Perl. Tell them they can write dynamic web pages. Tell them they can set up small Internet servers. Tell them their scripts don't have to use email anymore for notification, they can open a port and send the data directly into their alerts system.
Or, if they come from a hard core korn shell/C environment, tell them they can do in Perl most of what used to require C. And since Perl is an interpreter, you develop code faster.