More of a people one.

Simple matter of payback for the investment.

The MF keeps on getting cheaper. Every 3 years
we get a new one, and it costs less than the old
one while running a bit faster.

We have a huge amount of legacy code that would
cost FAR more to move to something else.
We have dozens of people who know and love it,
who have been running the business for many years.
We have many "systems", groups of programs that
run on it.

My job is to grab the low hanging high CPU stuff
and port it to Unix, but that will just leave
more CPU cycles for the rest of the stuff that will
ALWAYS run on it.