That was just an example. You could also pair the key to a person by some other method, such as storing a copy of it on a storage medium other than their phone.
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.