“With subscription products, you need to make sure the value promise is repeatable and there's a new chapter every so often that brings the user back. Early Fitbit had a decay curve to the marginal value — if all you're doing is telling someone how many steps they walked, there's a ceiling. Consumer subscription businesses only really work if they have really high long-term retention rates.”

The repeatable-value problem kills subscription businesses: recurring billing requires recurring utility

Many subscription apps have a 'first month delight' problem: the product feels transformative in week one and table-stakes by week eight. Carter's repeatable-value test: does the app have new chapters (new content, new insights from accumulating data, new social dynamics) that justify month-12 billing as much as month-1 billing? Content apps solve this with fresh content; tool apps must generate ongoing insights from user data.