Renaming files in a folder the app has access to should be fine in a sandboxed environment.
The app just has access to a specific folder in user directory and that's that. The issue for transmission is maybe that it allows you to upload files from a folder anywhere. But even that could be restricted via an Api in a sandboxed envrioment.
What iOS has taught us is that devs by default want access to things they don't need generally. They want an easier life so they would rather have everything open.
However for 99% of tasks there are ways around it. There are very few programs that can't or haven't been made in some fashion on iOS that exist on the mac. Most things that aren't on iOS will be down to policy (I.e. No file sharing etc..).
Apples reputation on the mac rests on the fact that it's easier to maintain and more secure by default. I'm sure apple will do whatever it takes to defend this. I expect in the long run all mac apps will be sandboxed by default and users would have to explicitly allow access to the file system for an app.
And no app should have access to system wide encryption. What app needs that? You can't stop people breaking into a server and changing files. But you can stop what that app is allowed to do on a system.
