"There's a very strong financial incentive for ad-powered search engines to keep SEO spam out of search results" No. Shitty results keep people exposed to ads

"There's a very strong financial incentive for ad-powered search engines to keep SEO spam out of search results" No. Shitty results keep people exposed to ads for longer, on average. 'A publicly run search engine would not have those incentives and would be at if anything graver risk of a "tragedy of the commons" type scenario where the engine is overtaken with spam.' You might be pleasantly surprised by the main work of the 2009 Nobel prize winner in economics, Elinor Ostrom's Governing the Commons.