The issue here is that a user that paid for your app many years ago continues to use up sync resources without providing any additional revenue. Maybe you'll be

The issue here is that a user that paid for your app many years ago continues to use up sync resources without providing any additional revenue. Maybe you'll be able to convert enough new users to subsidize the cost of the existing ones, but maybe not. I think the alternative of issuing regular paid upgrades makes more sense for productivity apps. You get recurring revenue, but it's tied to an incentive (new features) to pay the upgrade cost. The only problem is that the App Store doesn't really have a great method for issuing paid upgrades. You basically have to release a whole new app.