Looks like you know nothing about mainframes and "aged technology". I work with mainframes. zVM, DASD, DirMAINT, RACF and other buzzwords are in my resume, along with Linux, Java, PHP, XML, jQuery, MariaDB, HTM5, Eclipse and others.
Mainframes are not aged technology. They are perceived as such by small companies and people. Big companies with big bucks know a lot about mainframes. They know mainframes are the most reliable hardware platform on the market today, and I guess it will continue as so for a couple of years, because mainframes were made from the start to be reliable. Other platforms got they reliability implanted on them. Mainframes were designed reliable and resilient.
Mainframes today runs Linux too, not only the "aged mainframe operational systems." And here we have mainframes running hundreds of Linuxes with jBoss. They are about to be orchestrated by OpenStack, so managing all this "aged technology" will be done in brand new Android and iOS tablets.
Job prospects in my area, at least for the next decade, are very good. Half the openings in my area are still open, paying for a intermediate zVM administrator almost twice what a senior Java programmer or MCSE will receive. And there's no people applying!
But if the mainframe job market have a problem, is lack of people. Mainframes are not user friendly, and youngsters are not likely to devote two or three years learning something from the grannies, on a very harsh learning environment, with a step learning curve, when all their peers are talking about creating a new app and selling to Google for a gazillion dollars.
Peer pressure is a greater force than job prospects. I faced this pressure when I talked to my peers that I was learning mainframe and everybody laughed at me. Now I earn 3 times what they do, and I am training some of them to work with me.