If this were true, then there would exist no search engines consistently capable of finding content highly relevant to the user's query -- as opposed to just returning promoted content, which according to your premise is always winning the adversarial battle.

Sure, there is an adversarial element, which means there is a constant 'battle' for 'fairness' among results (or whatever your goals are as a search engine). But in reality, search engines as a whole seem to work rather well at finding what we want. Of course results are biased from adversarial pressures (and that's what we're discussing on this thread), but it does seem possible for "benchmarks" to work well in an adversarial environment.