[zulln]

Requiring a external storage medium would kill the service. I think you have to separate a service made for the masses and a service with focus on security/encryption. For WhatsApp there will be some instances where you have to choose between security and convince, and they have choose the former, which is only naturally.

[Veratyr]

I didn't say it has to solely reside on the storage medium. The phone can keep a copy and a user can make a backup.