[the_af]

It is indeed a conundrum.

Working in maintaining and supporting legacy software is both an absolute necessity for the business and a possible career dead-ending move for you. You're at the same time doing important work -- if not particularly interesting or groundbreaking -- and signalling to your employer "I'm not marketable enough that you must raise my salary or risk me leaving".

Working on legacy software can also be a dead-end when interviewing with some startups. Interviewers from MuleSoft once told me to change jobs and work on more interesting, high-profile open source tech stacks before interviewing again with them. (I did, but didn't try another interview with them because I later learned what they do isn't particularly interesting, either).

Some people do specialize in obsolete software and make it their niche, but I find in my country (not the US) the degree of success with this is overstated. For example, I know very few COBOL programmers who earn a lot of money. Most are in a lose-lose situation, earning average money and... working with COBOL.

[Spooky23]

The upside of being a COBOL jockey is that you have a steady job and basically just work a runbook. Some people dig that!