However, that developer investment is required if they plan to keep selling their application - hardly anyone will buy an app outright without modern platform s

However, that developer investment is required if they plan to keep selling their application - hardly anyone will buy an app outright without modern platform support. If you are growing your market, you can make your app a flat charge. If you are maintaining and and expanding an app for an existing user base, subscription is a better way to go (and something Apple does support well).