I have a thought, but I'm not well equipped to model the economics of it. I want to investigate it further.
When your cell phone can't be trusted - maybe the NSA has put on a virus, or another app - then you're in trouble. How can you be sure your data is not being modified? One way that Google and Apple accomplish this is through cloud sync. If your data is synced to the cloud, it can be modified, but it goes via Google or Apple, with logging included.