To be fair to Microsoft, this was a local model too and encrypted (through Bitlocker). I just feel like the only way you could possibly even try to secure it would be to lock the user out of the data with some kind of separate storage and processing because anything the user can do can be done by malware run by the user. Even then, DRM and how it gets cracked has shown us that nothing like that is truly secure against motivated attackers. Since restricting a user's access like that won't happen and might not even be sufficient, it's just way too risky.