The existential risk is that the incredible repository of nearly all human knowledge that is the internet will be flooded with so much LLM-generated dreck that locating reliable information will become effectively impossible (alongside scientific journals, which are also suffering incredibly under the weight of ML spam).
The existential risk is that nobody will be able to trust a photo or video of anything because the vast majority of media will be fabricated.
The existential risk posed by AI is that we as a species will no longer be able to transmit and build on generational knowledge, which is the primary thing that has allowed human society to advance since the end of the last ice age.