> Unfortunately there's just not a lot of value to being able to put on your resume that you're an expert at an older language that nobody has heard of, built on an in-house framework that will never be used outside of your current company.

I think this is, fortunately, only approximately true. Most of my career to-date has involved exactly this kind of work and it's certainly not true that there's no value in it, even on a resumé or mentioned in a job interview. And certainly some non-zero amount of people derive considerable value from exactly that kind of experience as some non-zero number of employers or customers need someone with it.

And, in a lot of ways, working on legacy systems can be both immensely fun and rewarding. Even just incremental improvement in or around such systems can itself be immensely valuable, e.g. setting-up automated builds or deployments, adding integration tests (to make future refactoring easier and safer), or rewriting or replacing a key portion of the system using newer tools or with a better design made possible due to the wisdom accumulated by a system performing real work over a significant period of time.