Spotted in the latest beta release, a new WhatsApp option could soon let you log out of the service without needing to delete your chats or data. https://www.notebookcheck.net/WhatsApp-is-working-on-a-new-option-to-let-you-log-out-without-deleting-data.1026749.0.html
What Whatsapp needs to do is give us cloud-based chat storage like Telegram.