I will say I have no experience in the ad space, but surely the SEO/ad companies will figure out how to game LLMs to make their sites more likely to be picked up by it, no? Or OpenAI would just directly sell ads themselves.

Yes - we're in what I like to call the Socialist Phase of AI (user acquisition). We were once in this phase with Google. Eventually we'll move into the as-yet-unnamed-by-me phase that Google (and search in General, also the internet) have been in for quite some time, where they try and squeeze out all the money that they put in during the Socialist Phase.