There are other bases for compensation. Conversions / actions / clickthroughs.

Measure what you can see. Don't obsess over what you cannot.

Good point, I had this question earlier about ads in general, but this didn't even pass my mind.

Idea: ad network w/ a daemon the publisher installs in their infrastructure between the users and the content. Maintains a blockchain which verifies exactly how many impressions there have been.

OP probably means major publishers who can solicit advertisers more directly.