Weirdly enough, this isn't necessarily true. Articles keep popping up about a shortage of FORTRAN or COBOL programmers. Yet when you look into it, they pay comparatively poorly... which explains the "shortage". I think this might be because there's still enough retired folks willing to contract a bit on the side. (And FORTRAN is not so bad.)

On the other hand, you can definitely demand obscene hourly rates for old, clunky stuff most people don't want to touch. Sharepoint is one example I know of. (Not to imply CF is clunky, but the worse the workflow, the higher the rate, obviously.)