> And what if what I want to do goes against some corporation's terms of service or whatever? What if I want to monitor what apps send over the network? Reverse engineer an app? Copy and manipulate their data? Automate tasks? Fake the personal information returned by system calls? Block advertising? Block surveillance?

You can do all of that on Android, many of those without root. If you want to modify any process's memory, you need a permission that doesn't exist. With sufficient imagination, you could invent a permission system that allows you to do that but doesn't allow a malicious app to do that. If instead you allow arbitrary tampering like you want, up to and including replacing the system partition without indication, there's no way for the bank to know your banking has been hijacked by somebody other than yourself. Your bank doesn't give a damn what you do with the banking app on your device. It cares if it has to deal with mass exploitation of its customers.