This isn't an assessment of content quality, it's of a user's experience. All Google needs to do is optimize for things like time on page, CTRs, etc to see use

This isn't an assessment of content quality, it's of a user's experience. All Google needs to do is optimize for things like time on page, CTRs, etc to see users abandoning those with paywalls and not those without to decide one is a better experience overall than another.