Cobol code isn't esoteric, but it is designed in a way that is unambiguous and clear about its control flow, but very verbose and tedious to read/write. Translating from Cobol to a modern programming language is trivial for LLMs for that reason because the lack of any hidden state functionalities like generics for example.

Modern Cobol has most of the goodies, it is much more feature rich than Go.

Your last paragraph is getting at the core problem: COBOL _is_ easy to understand and write at the language level but what keeps people on these systems is decades of accumulated business rules, and it’s almost certain that there’s no accurate spec for the whole system. You might have some overall documentation but I’d expect to find cases where someone added a special case back in 1987 to handle an edge case and everyone who works in shipping knows there’s something special to do if this usually numeric field starts with a $ sign etc. but it’s not described in some kind of spec.