If you trust the OS, it could hypothetically be built into the OS such that the app only gives what needs to be encrypted to the OS and gets back an encrypted payload to send, then the app wouldn't need to access your keys?
Of course at that point the OS has to either provide good key management itself, or a good API so that the apps can still make things seamless while still not accessing the keys directly.. and probably other problems I haven't thought of.
