If the alternative to a x% chance of upselling someoone to a yearly subscription is "read one article and maybe never come back to their site" then the price that the vendor will want to charge for that article is x% of that yearly subscription. You mention 1/2 a cent in multiple comments, but that's not a realistic expectation - if the paywalled sites would offer a per-article payment, they would definitely want to charge much more than that for that single article, closer to a dollar or so. They don't offer per-article pricing because the currently paywalled sites can earn more otherwise - the micropayment challenges aren't relevant, they will not sell their goods so cheaply (as it would devalue and 'cannibalize' their main revenue) even if there was a zero-fee micropayment mechanism.

Half a cent per article is a plausible target for all the weak/cheap sources who can't possibly afford to put up a paywall right now because nobody would pay. It's not going to be used by the currently paywalled sites, their business model relies on getting a not-micro amount from each customer.