Answering the titular question, why would the advent of AI-found vulnerabilities substantially change one’s threat model? Before LLMs, the prospect of an undiscovered vulnerability was non-zero probability then, so what exactly has changed now?

In terms of attacker capabilities, it would be a mistake to solely observe that attackers now have LLMs and conclude that this means the threat landscape has changed. The use of LLMs does not magically create a root escalation where one does not or cannot exist. Part of your threat model should include defense-in-depth mitigations, so that when (not if) an attacker finds a 0-day exploit, they are severely limited in gained functionality on your machines. In the simplest cases, jails or containers serve to keep separate the web-facing processes. VM isolation would be a further step, because an attacked cannot gain machine control without both a software exploit as well as a VM escape route. People hosting services can and should expect that their front line defense will fall, so they better have more defenses in place.

Phrased another way, the people who have done their threats analysis properly and have accepted the residual risk of hosting their own services that face the web, should not really be additionally perturbed by AI advances. This isn’t the same caliber of landscape change as would, say, the invention of a cryptographically relevant quantum computer. If the worst thing that can happen to your home IRC server is that it gets flooded by a script kiddie, AI does not mean your server can now burst into flames.

Much like with human hygiene, some basic rules are unusually effective against most threats. The equivalent to washing one’s hands is to stay on top of system and software updates. Institutionally funding the organizations (eg PHP Foundation, curl, USA CDC) so they can fix these problems would also help.