They would, but the app developer wants all their users to move to the new payment service (e.g. so that the developer gets to know more about its users) That

They would, but the app developer wants all their users to move to the new payment service (e.g. so that the developer gets to know more about its users) That calls for making it as easy as possible to migrate. Downloading a new app and deleting the old one is harder for users than using the iOS auto-update mechanism, so developers would prefer to have the ability to update their app with one that offers a subscription through a new payment service. Developers can force users of the old app by retiring it, but they can’t force them to download the new one. Chances are some users will think “what do I get out of this app?” and not download the new one. That’s the “losing subscribers” part.