You’re assuming a customer will never churn. Customers churn, and if I can get you to pay me a “lifetime” purchase that is more than I’d expect; then I come out

You’re assuming a customer will never churn. Customers churn, and if I can get you to pay me a “lifetime” purchase that is more than I’d expect; then I come out ahead. There is a ceiling with subscription purchases (provable with simple math: new customers * churn percentage * subscription cost = new money ceiling) but no theoretical ceiling with one-off purchases — it’s only limited by your marketing reach and funnel.