This is the crucial issue, but I think price discrimination by user-persona is way more downstream. Even assuming there's a sufficient population willing to pay

This is the crucial issue, but I think price discrimination by user-persona is way more downstream. Even assuming there's a sufficient population willing to pay, I believe the challenge is, how does the publisher value its content v. the user in terms of willingness to pay. And how does the user value content before they've been charged v. after they've been charged. And as you stated, how does content value vary within a publisher's content catalog. Viewability and ad slots (crudely stated) was the way that ads tried to solve the latter, but would users feel similarly? Especially since this created loads of paginated content such as "top 10 lists".