Banks remain with COBOL because they have a fuckton of COBOL code and 4 people can write it. There's nothing more to it.

Now, 4 million people can write it.

I don't think learning how to write COBOL was ever a problem. Knowing that spaghetti codebase and how small changes in one place cause calamity all over the place is. Those 4 people's job is to avoid outages, not to write tons of code, or fix tons of bugs.

In my experience, learning COBOL takes you a week at most, learning the "COBOLIC" (ha ha) way of your particular source base will take you a couple of months, but mastering it all including architecture will take you a year, half a year if you're really good.

One year from zero to senior doesn't sound that hard, does it? Try that with a Java codebase.

I would say more significantly, 4 million people can read it. The changes required for any given quarter are probably miniscule, but the tricky part is getting up to speed on all those legacy patterns and architectural decisions.

A model being able to ingest the whole codebase (maybe even its VCS history!) and take you through it is almost certainly the most valuable part of all.

Not to mention the inevitable "now one-shot port that bad boy to rust" discussion.

I would have you nnowhere near my production

It's got to be a lot more than 4. Global Shop (ERP) is written in Visual COBOL.