In COBOL implementations, it's generally not just knowledge of the language that makes you valuable, it's knowledge of the implementation at that particular organization. I'm not a COBOL dev myself, but I work with them, and part of the challenge is that everything is so uber-customized, tightly coupled, and there's 40+ years of undocumented business logic buried in the code.

It's like the old joke about the engineer being asked for an itemized bill: "Chalk mark: $1. Knowing where to put it: $4,999."