How is bringing down the whole power grid in any particular country an extinction level event? I'm pretty sure that even in the worst case scenario it would be like a month of chaos in one particular part of the world at most, hardly something that would have a long-lasting impact on the humankind's ability to survive at large.

If the answer is "they'd at least try to nuke the country that did it in response", then once again, LLMs are not the main threat.