They're leaving no way out, and making things like "Syncthing" and general file managers really hard to continue operating.
That's becoming the answer with everything on Android, for anything useful you want to do, you need to root the phone. The stock phone is becoming just good for running Facebook and Candy-crush.
A communication-only device that can take photos and browse the web is very useful, but I think it will lose market to a proper computer every time.
It might not. Every phone company has strong incentives to optimize their offering to the lowest common denominator. Majority of people are not tech-savvy, and they have no context for what computers are capable of and how those capabilities could be applied to their lives, so they simply choose from what's being offered by hardware and software companies. People who care about this stuff are a minority, and there's not much money in catering to them.
If computers could be bicycles for the mind, the modern trend is making people sit on their ass instead, and consume what's delivered to them.
Because files don't always have a clear ownership. I have a folder full of photos and I don't want any particular application to "own" them. That a file corresponds to a concrete app that owns it is a lie, that's being pushed hard by mobile OSes.
And that's not even getting into the security issues with a shared filesystem, where a single evil process can access all your data.