Its not the learning of COBOL thats the issue - its the custom nature of a lot of older systems and understanding the associated business logic.

I remember getting taken thru the core interconnectivity of the mainframe at a bank and being shown a diagram of the key flows which didnt look to bad. They then overlayed all the flows and the screen was like a bowl of spagetti - and understanding that is where the real skill and expertise is

Thats probably true of most legacy systems