The writing was on the wall.

The irony is that the premium price was marketed as a guarantee of perennity.

This must serve as a reminder that third-party cloud storages have a price as well as a maintenance price. Moreover, iOS APIs and devices evolve a lot and ask porting work with every new version. Additionally the more you implement your own custom UI elements the more you expose yourself to additional work when porting to a new device or for the next release. That's certainly why not only the syncing part is shutdown but also the app is not ported to iOS 10 and is removed from the app store.