Perhaps you just need to wait 10 year for everyone to retire and then the market supply will jack up these rates!
There's a good argument for unionizing here. There are few enough COBOL devs that they could demand what they are worth and support it with industrial action.
This will be the fastest method of getting bank software rewritten in rails.
As a younger person (graduated 2012) who entered the mainframe space for my first job then left after 2y, I can very much vouch that compensation for software devs working on zOS systems writing COBOL or HLASM is peanuts. You'd be looking at 1.5-2x'ing your income even if you took a job at a series A startup writing CRUD apps in Rails/etc.