It's true that searchers don't pay Google money, and advertisers do. But Google is running a platform. In the past I've compared it to an information marketplace. And the goal for Google is to make the market run as efficiently as possible, otherwise they risk losing one side.
Searchers don't pay Google, but they do (presumably) pay Google's advertisers, who pay Google. If you lose the searchers, you lose the advertisers.
Now of course there is a balancing act, which you allude to in your last paragraph. But there are plenty of easy examples where Google returns no ads even though they could. A search for "how old is barack obama" just returns the number (or Wikipedia), without ads, even though I'm sure there are advertisers out there who would pay for an ad to be shown.
So obviously it's not universally true that "the best search results Google can produce are those which maximize their revenue." Perhaps adding some subtlety to your argument would help me understand exactly what you're saying.