alt Hacker NewsSure, I know for a fact that what you're describing exists. That's not really what I mean by new business being built on it. That's a case of a very large and old business already being so locked into the mainframe ecosystem for their core systems that anything new they try to do ends up needing some kind of integration system with the legacy system.
What I mean is that nobody starts a business today and says "Ok, we need an IBM mainframe running DB2 and we'll have a bunch of COBOL, ReXX, and PL/I programs for handling our business logic".
There was a decent amount of that going on in China in the 90s and early 2000s actually in the banking sector. You probably won't see much "new business" until you see companies in new large markets explode in size. As in you are unlikely to ever see "new business" in the US, because you'd need a new bank or something to somehow explode to the size of one of the big four and suddenly realize they need to get on board with what all the competition is doing in order to compete.
But it has happened at least a little within the past couple of decades, most notably with China but there have probably been other examples in Asia.