net monthly churn rates below 5%
That's low for a lot of subscription businesses. It seems that one metric he's not really looking at is the amount you can get for a one-time up-front purchase, vs the amount you can get for a subscription
I bet the churn on stock photo sites is high. We have one where we pay the monthly fee, but then periodically we'll binge and sign up for another to get some other images and download as many as we can in the minimum subscription period, then cancel. The thing is, our alternative is often something like, say, $40 for the images we actually have identified and want, and $99 for the minimum subscription. So effectively, we end up paying $59 more than we would have if we only had the a la carte option.
Aside: a friend who has been a reasonably high-level manager at some giant corporations always tells me that the most important metric for measuring a manager is churn because it has a huge impact and, unlike other metrics, is often more closely tied to the manager than, say, the profits that group made (if it can even be measured on the group-level).