My partner makes a living writing z/OS assembly language, has been for many years now. The platform is still going strong as a business. The main problem they face is all the folks who know how to program these things are retiring (or dropping dead at their keypunches.) It's very hard to convince new people to learn how to operate these systems.
He does system level stuff, enhancements to IBM's z/OS. A lot of their customers are big financial institutions, banks and the like that use MVS for transaction processing. His software augments the operating system and isn't application specific.