Here's my thought experiment: nothing changes.

I have literally never used it. I have one very simple rule for chat apps that I hold to over everything else. The user identifier must be a username or email address. It must absolutely not be a phone number or something else that intrinsically ties it to a specific device. What's App has failed that test since before Facebook bought it. In a world where we have multiple devices and move between them often, it has always been insane to me that other people don't think there's a problem with using a phone number as a unique identifier for an individual. And it only gets even worse when you start adding international travel, changing your phone number, etc. into the mix.