I do not get why you say that COBOL programmers get paid less. Once you know everything about the systems and the language you should be able to get above market pay because you are really hard to replace. Pay me more or I leave would be a pretty easy management decision.
“Should” is the key word. I’ve looked at COBOL job listings every few years and I’ve never seen one for more than $90k US. So maybe once people are on the job maybe they have more leverage but those job listings don’t indicate any desperation for talent.