Fear of not keeping up with the Technical Joneses is a huge influence in our industry because everyone sees "old" programmers booted first when a company or the economy sours. Nobody wants to be That Guy (or Gal).
If mainframes are perceived as a shrinking or outmoded technology, obsolescence fear kicks in and people avoid it. It may need some clever marketing to trick people into giving up that fear or impression. BS works in other areas, per mind-share. Call mainframes "high-up-time cloud", Cloud++, cloud.js, Deep Cloud, or something. There's better BS'ers out there than I.