Worse-case scenario, AI finds a way to mine Bitcoin on its own and manages to control a sizeable portion of the network's hashrate (raising the risk of a 51% attack). But this is very unlikely to happen anytime soon. Bitcoin is an open source cryptocurrency, so it'll be easy to tweak it against AI threats. With a community-approved hard fork (or soft fork), Bitcoin will remain as secure and reliable as usual. Not even Quantum computers stand a chance against Bitcoin.
That's not how AI work. AI cannot magically produce compute capabilities, or have the ability to mine Bitcoins, or do anything of that sorts. AI, as of now is very much misunderstood, and they are not like the Terminator movies. AI (or rather LLM as you know it), are designed on a huge corpus and are just fancy math. There is no chance that would take, over the world, or something.
AI can design chains of its own. But will they last long enough? If there's lack of interest/demand among people, AI-generated chains won't survive. Even if they do, how would they pose a direct threat against Bitcoin? It doesn't make any sense.
Can AI really design chains of its own? Or are they just a variation of a particular/combination of blockchains that we already have?