True, but those people have heard of IE or Safari, or some other major browser, no? It'll be rare that someone will publish a website without trying it out on at least one major browser. If all major browsers strictly enforced validity, then people will be forced to create valid HTML, or at least their authoring programs will be forced to do it.
But if all major browsers enforced validity strictly then the author would be forced to fix the problems before publishing their site (assuming they test it on at least one major browser).
I agree with you that this would never happen, the net is too unregulated (and rightly so) for one entity to enforce anything beyond the minimum requirements. If one search engine gets picky, we all switch to another, it one browser gets picky we all switch as well. The only way is if there's a concerted effort by all major players, and then I believe it's better to do it at the browser level rather than the search engine level, and only for future versions of HTML so no one has to go back and fix existing sites.