WhatsApp includes a feature to explicitely cope with users losing their phones or the phones being broken. That feature is discussed in a article from the Guardian where it is seen as a back door:
WhatsApp has the ability to force the generation of new encryption keys for offline users, unbeknown to the sender and recipient of the messages, and to make the sender re-encrypt messages with new keys and send them again for any messages that have not been marked as delivered.
The recipient is not made aware of this change in encryption, while the sender is only notified if they have opted-in to encryption warnings in settings, and only after the messages have been re-sent. This re-encryption and rebroadcasting of previously undelivered messages effectively allows WhatsApp to intercept and read some users’ messages.
[Whatsapp justifies that to cope when] a contact’s security code has changed. We know the most common reasons this happens are because someone has switched phones or reinstalled WhatsApp. This is because in many parts of the world, people frequently change devices and Sim cards. In these situations, we want to make sure people’s messages are delivered, not lost in transit.
That indeed allows you to recover your chat history in your new phone without any access to the old key. But that also mean that it is easier to steal WhatsApp exchange that end to end encryption should allow. And it also means that WhatsApp can actually give the full history to legal authorities if they were asked to, simply by simulating a new device...