This topic is pointless. Its not remotely likely this could ever happen. Only way for it to happen would be to have World War 3 with something highly unlikely as all the various server farms being nuked all at once deleting all the rolling backups and replications. Its not going to happen, ever.
But lets just roll with it and let say it did happen.... They would simply give all players level boost and max level characters with pretty much everything unlocked and hope you will stay and enjoy Battle for Azeroth.