On the plus side, the sheer power consumption of using an LLM to dish out targeted advertisements will be prohibitively expensive. Any agency stupid enough to do this with current technology is gonna go bust.
As for hosting the LLM locally on the viewer's machine... Remember the furore of shady companies burying crypto miners into their software? This is going to be even more wasteful of system resources and is going to result in such a sluggish user experience that the industry will go bankrupt.