Can't it be device to device , once you get to new device you mark that device as add and as soon as u login it can sync.
But i found whatsapp is next to mp3 in my and my friends phone so it is not easy from bandwidth perspective
This way, you might not have the full conversation on each device - e.g. you write "a" to alice on your phone, turn it off, turn your pc on and receive "what do you mean?" from alice - the conversation is otherwise empty.
That's not a good UX.
You'd have to store the chat history somehow - and thus loose the privacy aspect of not storing it.
Being able to sync across all your devices requires they start storing all of your communications centrally, which defeats the whole information security model.