I asked him point blank at one point, what the average starting salary for a graduate from his school hired to do mainframe work was. The number he threw out was less than 1/2 of the starting salary of the average comp-sci graduate from the state school down the road. Granted the people from the CC had a two year degree, and were being hired for pretty low level positions. But unless the pay ramp was really steep anyone doing it was crazy not to just enroll in the comp-sci department at the state school and put in the additional two years.
I considered if becoming a mainframe expert was a good career move, and from what I can tell midlevel mainframe guys don't make as much as midlevel PHP experts. The top end is ok, but i've heard of people doing vmware designs/installs for more money. Maybe some mainframe guys are paid by the gold bar, but I sure didn't meet any.
That is probably why there is a "shortage". People my age look at it and decide its not a good move because while companies will pay 8 figures for a machine they won't pay a fraction of it to the guys running said machine.