> DuckDuckGo appears healthy and quite content with making money off of keyword-based ads alone.

This makes perfect sense. Show an ad related to what a person is interested in and looking at. When I did adsense stuff a long, long time ago, I seem to remember that Google would request a copy of the page, and then tailor the ads to the content of the page. There was absolutely no interest at all in "Who" was looking at the page. It seems to me there is still a tremendous amount of money that could be made with only that use case.