If it ain't broke, don't fix it. So letting an LLM loose on a mainframe is like letting a fox into a henhouse. :)
The problem here is that people who understand these systems are all retiring. Majority of the devs are over 60 and there's simply not enough new talent coming in to replace them. So the real challenge companies are facing is will there be enough people to safely maintain these systems in the next decade. If they do not, it means failures in credit card systems, airline reservations, insurance claims and more.
The age of mainframe folks is highly bi-modal. The original folks are dying of old age. There is a new crop of 35-and-under folks that have sprung up as they noticed that the work is very steady and the pay is good. Between ages 35 and 65, there aren’t many mainframe people.