I am on an iSeries, the little brother to the z. We have a pair of zSeries as well. COBOL and RPG (its fully free form by the way, looks like any other language) are both very much rooted in business logic, in particular math. Their number handling is well known and without inconsistencies of many newer languages. That and the code base is well established and works. Throw in that memory leaks are nearly non existent, code is independent of changes to the microprocessors and your investment always comes forward.

Seriously, we have code whose originals can be traced to systems long dead. Yet code is not stagnate, full SQL integration exists for most language, there are many ways to get to the web. That and you would be surprised at the number of companies who use these machines simply for one reason above all, reliability. Our last crash was a site failure and even then we rolled to the DR site instantly.