I highly doubt that those gigs will exist in the future, or will become even rare than they are. The big banks and financial companies are making a decent effort to get rid of those legacy systems (but are on decade + time scales). They also are weary to even hire new people in those stacks, and are increasingly relying on contractors through places like TCS that will put people through bootcamps. I know several places that have the policy of no new cobol code, only maintenance. These people with the ludicrous salaries, I assume are consultants, or have even more niche specific and rare skills beyond just COBOL and JCL and RPG or whatever, probably with specific, ancient implementations on ancient machines.