I think you're assuming more of my answer than what I gave. That's fair given that this is the point of the article, but it's not mine. I'm very specifically only responding to "is there a per-user marginal cost on software?", and my answer is most definitely yes.
To warrant a recurring payment business model, I think the right question to ask is "Is there a per user-year marginal cost on software?", and now the answer is in my view, much more complicated and domain-specific. Worse yet, I think that there's perverse incentives at play here in recurring payments.