It's not like the Bank of Americas of the world are going to get an Anthropic or OpenAI API key, and then use the prompt "move this off of COBOL please". They'll either hire a contractor who'll work out how to best utilize AI to migrate the existing codebase off of mainframes, or spin up an internal team to do the same thing.
I've worked on the type of legacy IBM systems being discussed here. They're often filled with absurd hacks. VMs running outdated operating systems on emulated hardware because that was cheaper than updating the application. Systems so confusing and poorly documented that only a single employee knows how to update it. Known bugs in software that are solved by pre-processing or post-processing inputs/outputs to hack around the issue.
"Vibe coded slop" isn't going to be any worse than "IBM-outsourced slop" or "Maintained by one quirky IT guy for 15 years slop"
.... and that's because the mainframe industry is so heavily gatekept that the information feels practically word-of-mouth between a circle of old greybeards that don't want to let the kids touch it.
Always wanted to even get my hands on ZD&T but IBM never thought I was worthy enough.