If the code still fit the problem then you wouldn't need to pay a cobol programmer. If its hard to change the code because its too expensive to change, then you have tech debt.
But it's only debt if business decisions mean you must change it. I've seen far too many programmers crawl through legacy code that need not be replaced (it does its job) merely because it's using an outdated technology. But if there's no business justification for that, I'd argue it's not debt at all. There is absolutely nothing wrong with some ugly COBOL code doing its job with no current need for modification. That's not debt. Debt is something you have to pay back.