I worked at IBM for 7 years and I worked with mainframe and AS/400 systems while I was there.

If you want to learn Mainframe and COBOL for fun I say do it. I did enjoy working with those technologies and you probably will too.

However if you want to do it because you think you'll make lots of money, unfortunately that is not the case. Yes I know you hear all the time about how COBOL programmers make bank. That is not quite true. They are not looking for people that know COBOL or Mainframe. They are looking for people that know their big ball of mud. If you know their big ball of mud they'll pay you a lot. However if you just know COBOL and they have to train you on their big ball of mud. Well they would rather send that work offshore to pay the lower wages. Any work done onshore is paid at low rates.

I haven't touched those systems since I left IBM 15 years ago. In fact I don't even have it on my resume anymore. Cloud technologies pay so much more and the jobs are easier to get.