I enjoyed my decade working with mainframes probably the most out of anything in my career, but I suspect my opinion would be different had I been an application developer forced to work in COBOL.

There's a lot of very cool and technically interesting stuff to explore on the platform. Tons of great computing history, you really feel a bit like you are peering back in time going decade by decade as you move inward towards the core of the operating system. It's helpful to remind yourself that it's a direct descendent of the first successful commercial virtual memory implementation, and the core of the operating system has pieces that predate that. You can see the outlines of the world before that and how things adapted afterwards, how the modern process model slowly evolved out of DAT and time sharing, how things were handled in a fully vertically integrated system before the concept of device drivers was fully realized. How batch led to time sharing and transaction monitors and how all of that slowly turned into the web. And of course all the different tricks programmers used to be productive in that kind of a world.

It's impossible to fully realize all that we take for granted in modern computing when you haven't seen any of that.

Unfortunately the vast majority of people that work on mainframes never get to see that side of things.