Okay, but you're addressing only the ideal scenario of beneficially matching buyer and seller, where the product user is buffered by (honest broker) Google and

Okay, but you're addressing only the ideal scenario of beneficially matching buyer and seller, where the product user is buffered by (honest broker) Google and thus not yet revealed to the advertiser. But once you cross over (via clicking), whatever fraction of your digital identity is effectively sold to third parties who keep packaging your identity further upstream, in very durable ways. Now you exist in many data warehouses, who are eventually integrating the various fractional identities into a good picture. And it is no longer hypothetical to have those identities connected to your real identity. I do believe Google is an honest broker but we have no reason to trust third parties.