I think OP was referring to the case where new illnesses that are not part of the training set are never going to be diagnosed by AI.
Of course, that's not what's going to happen. :/
That's not how that would work in the real world. In a lot of places a doctor has to put their signature or stamp on a medical document, making them liable for what is on that paper. Just because the AI can do it, that doesn't mean the doctor won't have to double check it, which negates the time saved.
I would wager AI-assisted would be more helpful to reduce things doctors might miss instead of partially or completely replacing them.
I don't have a crystal ball, so this is a made-up scenario.
Sure, LLMs might not do this anytime soon, but once models understand enough biology, they're going to identify patterns we don't and propose new diagnoses. There's no reason why they wouldn't.