To a certain extent you have to ask what does a one-off purchase of an App mean any more. We got to a point on desktop where you could buy a windows XP application and it'd last forever, and that's fine. Or even a OS X application for several years. But mobile operating systems are still a treadmill. If you buy the app what are you getting? This version and no updates? That's not how iOS works. Will it support future iOS versions? Do you get the new features? Can you install it on a new device if you buy a new device? You bought it one off, if a new version of iOS comes out and the old app isn't supported what do you do?