I do the odd bit of mainframe security stuff now and again. You can argue the toss all you like about hercules, but the fact is that it's not up to scratch for

I do the odd bit of mainframe security stuff now and again. You can argue the toss all you like about hercules, but the fact is that it's not up to scratch for running a proper stack. Saying you can run Z/OS on it with CICS and RACF and DB2 and all the rest to learn is fine, but you try finding half of this stuff. The reason young programmers (and not so young programmers) aren't interested in mainframes is pretty simple, it's largely alien to modern computing users. Mainframes are rarely taught in CS degrees, most people go their entire careers without going near a mainframe and when they do see them, the fundamental concepts (batch rather than interactive, segregating security from the main OS etc.) they recoil in horror. It's also a highly specialist field. Mainframes are typically designed for repetitive tasks with extreme reliability, in the same way that GPUs are designed for specialist programming cases. It's unsurprising then that there isn't exactly a rush to fill the niche.