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.

Lots of government too. I worked at a government agency that ran it’s business off of mainframe. I worked there as a web developer and we had to consume data from the mainframe in our web apps. Very tedious process.

Yeah, forgot governments, so we can add one more to the list of 'nobody dreams working there as a tech person' list.

Yeah, the hiring process is often 6+ months long for half the pay of private sector jobs, and 1/5 the pay of the consulting companies they bring in to do the bulk of the work anyway.

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.