When you stop paying your developer account membership fee, the app isn't deleted from the users' devices - it is merely removed from the App Store. This means

When you stop paying your developer account membership fee, the app isn't deleted from the users' devices - it is merely removed from the App Store. This means that existing users who don't have a copy of the app on their computers will still be able to keep the app if (1) they don't delete it from their devices or (2) they don't try to restore from iCloud (which basically downloads previously installed apps from the App Store).