Agreed completely. But users will (eventually) run away if they do not perceive the search as efficient. There are now options in search. People use google exactly because they consider results good enough. If googling "photos from X" starts returning most pictures from Google-associated resorts and this gets coverage from first tier news agencies Google will see non-negligible defections.
I also do not think Google is likely to do aggressive search filtering, not because this is "not right" but for pure self preservation -- aggressive filtering would be cutting the branch they sit on.
> ...advertisers who operate on the assumption that the users see Google searches as mostly efficient and impartial.
I do not believe that is factually accurate. I think advertisers don't have a compelling reason to care whether or not searches are impartial for the reasons I expressed.
An advertiser might care if there were another, more impartial search engine that was drawing those eyeballs away, but that seems vanishingly unlikely given the current ubiquity of Google as a household name and company.
Sure, DuckDuckGo or whatever might draw some of the more tech-literate or paranoid crowd if Google started aggressively filtering results (which, for the record, I don't think they are doing or are likely to do), but why would advertisers care? Those people represent a tiny fraction of their eyeballs, perhaps so small as to be lost in statistical noise. Besides, many/most of the tech-literate folks use adblockers anyway, and the paranoids probably don't want to transmit their personal information online to buy things.