I just did a search for "cobol" on Australia's largest job website. It returned 87 results, and (from a quick skim) they mostly seem to be legacy maintenance positions in banks and financial institutions. Mostly distinctly better paid than more "modern" language based jobs - presumably due to the rarity of Cobol experience.
So yeah, it seems like Cobol would be worth learning if you 1) don't mind doing legacy maintenance and 2) you want to get into a niche which is well paid and probably not very competitive since it's something few people are learning anymore.
(I'm assuming the Cobol market would be similar in most First World economies, but could be wrong?)