But that's just it - Google's business interest is always about protecting revenue from search advertising, which is never simply aligned with the search user o

But that's just it - Google's business interest is always about protecting revenue from search advertising, which is never simply aligned with the search user or with search quality, but exactly as you describe it "a really complex system here with all sorts of incentives, variables, and issues on all sides". It’s perfectly reasonable to speculate about whether search quality is sacrificed in the interest of some incentive Google is trying to balance in favor of its own revenue stream. The fact that it’s complex is immaterial.