This sounds no different than the static analysis tools we’ve had for COBOL for some time now.

One difference is people might kind of understand how the static analysis tools we've had for some time now actually work. LLMs are basically a black box. You also can't easily debug/fix a specific problem. The LLM produces wrong code in one particular case, what do you do? You can try performing fine tuning training with examples of the problem and what it should be but there's no guarantee that won't just change other stuff subtly and add a new issue for you to discovered at a future time.