So, you can't fill a mainframe developer opportunity. Boo and also hoo.
Employers talk about how they can't find any people with mainframe skills and thus their vacancies go unfilled. People with mainframe skills talk about how few opportunities there are.
Of course the shortage is not a shortage of qualified applicants, it is a shortage of applicants willing to work for the compensation the employers are willing to pay.
Either compensation rises, qualified applicants adjust their expectations downward, or employers adjust their expectations of acceptable qualifications downward. Place your bets.
Remember who uses mainframes: Government (who have big stonking gobs of your personal data), Banks and other financial institutions (who have your money), Credit card companies (ditto), and Insurance companies (ditto).
Unfortunately the current coping mechanism to deal with this invented skills shortage is for employers to take the less qualified applicants, repeat until quality of service drops below an acceptable threshold and hope they're absent when the inevitable disaster occurs.
The race to the bottom is a mug's game, why are you trying to win?
Maybe let's actually give the next generation a chance to make up their own minds about mainframes instead of being repeatedly told how the technology is going away and there's no future in it. You know, people have had entire mainframe careers since pundits started predicting the demise of the platform.
Maybe make it clear during recruitment that your company doesn't engage in pointless paperwork or employ sociopathic managers.
COBOL is not hard to learn. Large chunks of what could formerly only be done with Assembler can now be done in high level languages, or C, or C runtime routines called from higher level languages.
Menu-driven ISPF is not your only UI for a mainframe. There exist Eclipse-based interfaces, and Zowe is making it possible to enhance and better integrate those.
z/Unix, z/Linux, SMF, Language Environment, these things are all actually pretty cool. Most people like working with cool stuff.
Maybe it's not the mainframe. Maybe it's you.