DB2 on a mainframe is not the same as the UDB or iSeries flavours; in fact, in some areas (eg. running stored procedures) it is quite different. It also offers some performance enhancements which are completely incompatable with UDB and iSeries DB2, so your comment about lock-in is not really valid. Depending on how an application uses DB2, the effort to port between environments can be considerable. You also fail to mention CICS and IMS, both in most likelihood used heavily by the people who make sure that your bank balance or pension balance is accurate every morning.
But all this is to miss the point. Readers who have honed their current skills on commodity hardware and software, using common programming languages and technologies, many donated for free from a community of other programmers, might forget that for many of us the only way into this business was to be fortunate enough to work for companies who could pony up enough (ie. < £1,000,000) for a mainframe, plus attendant controllers, tapes and disks.
The Hercules project has always been (IMHO) about allowing people to run mainframe operating systems on commodity hardware, thus allowing those of us who like to keep our IBM chops up to scratch, or simply scratching an itch, to do so; not ripping off IBM's IP to undercut their hardware business (as if a x86 and a z10 are remotely comparable).
IBM's stance on modern operating systems frustrates those of us who would like to keep the mainframe ecosystem healthy by being to contribute at our own expense, and in our own time.