I assume at some point the calculus flipped for ISPs to not annoy you with low-quality ads and make customers cut them out, but instead operate a real DNS resol

I assume at some point the calculus flipped for ISPs to not annoy you with low-quality ads and make customers cut them out, but instead operate a real DNS resolver so you can still sell their real-time browsing history to advertisers. Part of that is the Google and Cloudflare marketing campaigns for their public resolvers, so people had a ready alternative.