The issue I see is that for certain apps, such as one I am currently working on and hope to publish soon on iOS, is that they do require a lot of maintenance once published even if there were no server costs. Given the amount of work I already put in it and how much more will be necessary even just to keep the app correctly running in the future, I don't really see what other monetization approach would make sense for me. Actually, I would even argue that selling an app without a subscription might (sometimes) be setting wrong or blurry expectations: if a user accepts to pay today a single time, how long are they expecting updates for? Will it only be basic bug fixes or also major new features? With a subscription, I feel like at least if they are unhappy with my app, they won't really have lost anything and can just unsubscribe, since they had basically accepted, IMO, that the money they put in my app each period of time is only for the service and potential updates in that small period of time and not future changes.