No, not for the foreseeable future. In fact, this is the absolute hardest possible code translation task you can give an LLM.

COBOL varies greatly, the dialect depends on the mainframe. Chatbots will get quite confused about this. AI training data doesn't have much true COBOL, the internet is polluted with GnuCOBOL which is a mismash of a bunch of different dialects, minus all the things that make a mainframe a mainframe. So it will assume the COBOL code is more modern than it is. In terms of generating COBOL (e.g. for adding some debugging code to an existing system to analyze its behavior) it won't be able to stay within the 80 column limit due to tokenization, it will just be riddled with syntax errors.

Data matters, and mainframes have a rather specific way they store and retrieve data. Just operating the mainframe to get the data out of an old system and into a new database in a workable & well-architected format will be its own chore.

Finally, the reason these systems haven't been ported is because requirements for how the system needs to work are tight. The COBOL holdouts are exclusively financial, government, and healthcare -- no one else is stuck on old mainframes for any other reason. The new system to replace it needs to exactly match the behavior of the old system, the developer has to know how to figure out the exact confines of the laws and regulations or they are not qualified to do the task of porting it. All an LLM will do is hallucinate a new set of requirements and ignore the old ones. And aside from just knowing the requirements on paper, you'd need to spend a good chunk of time just checking what the existing system is even doing, because there will be plenty of surprises in such an old system.