> Obviously you will be able to see all your WhatsApp messages when WhatsApp goes down. The same is true for Facebook Messenger and many other applications, because everything is saved locally. It's even possible to send WhatsApp messages while being offline which will then be delivered later on.

I wonder how long this has been the case. My experience in the past is that these applications are not usable without a stable internet connection. That said, email clients have a concept of an outbox where messages that were sent are placed until an internet connecton is established. At that point, those messages are actually sent to their recipients.