TL;DR: Google is heavily stuffing more and more ads into every monetizable SERP, which explains the ever-increasing revenue.
But my theory (expanded in more detail here https://news.ycombinator.com/item?id=47957708) is that its cash cow is heavily optimized towards its current anti-competitive, multi-sided chokehold of the online ad market -- look up the AdTech Antitrust findings around manipulating ad auctions, e.g. Project Bernanke -- which relies on ad volume.
With agents however, I suspect that volume will shrink drastically even if they stuff ads at each step of the conversation. Because it's only the final click that will matter, and those ads will be meaningless.
It's not clear that Google can charge enough for that final click to compensate for the loss of both, the value of all the ads that lead to it, as well as their ad auction manipulation premium.