[jsudhams]

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

[JonnyaiR]

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.