I've yet to see anything that threatens Google's ad monopoly.
It's not that a dominant position goes away overnight. In fact that would be precisely the impetus to spur the incumbent to pivot immediately and have a much better chance of winning in the new paradigm.
It's that it, with some probability, gets eaten away slowly and the incumbent therefore cannot let go of the old paradigm, eventually losing their dominance over some period of years.
So nobody really knows how LLMs will change the search paradigm and the ads business models downstream of that, we're seeing that worked out in real time right now, but it's definitely high enough probability that Google see it and (crucially) have the shareholder mandate to act on it.
That's the existential threat and they're navigating it pretty well so far. The strategy seems balanced, measured, and correct. As the situation evolves I think they have every chance of actually not being disrupted should it come to that.