As far as I understand, the issue with Cobol specifically is that the code is riddled with gotos and global variables - and untangling all that mess is the real

As far as I understand, the issue with Cobol specifically is that the code is riddled with gotos and global variables - and untangling all that mess is the real issue, not converting into Java itself. Using traditional algorithms you end up with literal exponential complexity very fast. You also need a human's ability to figure out which new abstractions to create - otherwise you will end up with a code that is just as difficult to maintain as the original.