You have no idea. This is not some database field that you can just alter with one statement and update a field on a screen. Things like this get hardcoded into hundreds of thousands of lines of assembler and cobol and need to be changed and tested everywhere. With no automated testing or other tools to mitigate the risk. It is an absolute nightmare. And 5 or 10 years from now it will be way worse because working on a mainframe is a deadend career and very few people are learning it.

Having worked adjacent to some of these kinds of projects, can't upvote this enough. Not to mention that business doesn't take the suggested 1-2 years to simply document and define test cases for the old/new system to ensure such a new system works, and simply start working on the new system based on what the business/da types drew up. Missing literally tens of thousands of legal edge cases defined in the legacy codebase.

Which part is untrue? That there isn't enough personnel or that it's a severely limiting career choice? Because what you're saying confirms the first one, and for the second one, not a lot of people dream of working on legacy stuff in a big organisation, and those are your only choices if you're a mainframe programmer. Say in the US, for mainframe work, there are what, 30 banks? 5 airlines? 20 insurance companies? Not a lot of choice, and it's doubtful they'll offer "perks" such as remote work.

They are super wanted but it's true that it's very hard to get into.

Want to learn Windows, Linux, cloud whatever? Just build a cheap home lab and play to your heart's content. Because really, training is not how you get good. Nor is certification. It's the hands on experience that comes from running into the crap the vendor doesn't want to tell you. All the little annoying flaws they keep out of the documentation under pressure from marketing.

Good luck doing that with a system Z. Try to find one on eBay for peanuts the way you can get a nice ESXi server.