(There are two easy ways to backdoor end-to-end encryption if you control either the signaling or one of the ends.
The first, if you control the signaling, is to simply add another end into the conversation and not tell anyone about it. Any centralised messaging service can implement this.
The second, if you control one of the ends, is to do what Apple plans to do: look through the data after it has been decrypted and have the end take some sort action based on what you find.)