That little thought experiment is true for many online services, from social networking to (marginally) publishing. But nowhere is it more true than for search results, which differ in two fundamental ways: being text-only, they don't bother me to anywhere near the degree of other ads. And, second, they are an order of magnitude more valuable than drive-by display ads, because people have indicated a need and a willingness to visit a website that isn't among their bookmarks. These two, combined, make this the worst possible case for replacing an ad-based business with a donation model.
The idea mentioned in this readme that "Google intentionally degrades search results to make you also view the second page" is also wrong, bordering on self-delusion. The typical answer to conspiracy theories works here: there are tens of thousands of people at Google. Such self-sabotage would be obvious to many people on the inside, far too many to keep something like this secret.