The Internet is big, but one can do quite a lot of damage with ordinary bots and worms that exploit individual widespread vulnerabilities, which LLMs are perfectly capable of writing. Most of the damage also doesn't rely on hitting every long-tail website.
I'm honestly not that concerned about cyber impacts of LLMs relative to other impacts. I just don't like to see the whole concept of being worried dismissed as obviously baseless on the basis of one pretty shaky scale argument.