People tend to identify themselves with technology they mastered and miss out on all the improvements that happen. This works for a few decades if they're lucky, but someday they're old and wonder why nobody values their skills in an obsolete technology.
We can't all maintain Cobol code bases when we are 50.
I think it's also difficult in practice because it feels good to master things in general. It's a nice feeling to know that you know a system inside and out, so detaching from that is going to be hard no matter what especially if you've invested quite a bit of your life into it
In fact, a 50 year old is more likely to have caught the rise of desktop GUIs and of Linux than have spent a career doing COBOL.
It was a joke that targeted the argument: "You know that old tech is still in use? Look at all the COBOL used in industry XYZ."