That would seriously break the network effect and eventually kill them off. Skype, Yahoo Messenger, etc got popular because they where free and easy to use. However, the market is really fickle and network effects can kill stuff off really quickly.
WhatsApp is already DOA in my circle of friends, it might come back but the odds aren’t great.
The WhatsApp network effect is strongly tied to borders though. Almost no one uses it in New Zealand for example, but a lot of people use Facebook Messenger but it isn't completely dominant like WhatsApp is in some places. I wonder if it ties back to countries that have good value cell phone plans? Even the very cheapest plans here come with unlimited SMS and it's been that way since smartphones were first taking off.
I think you're understating the strength of weak connections. If I have even one friend who's not on Whatsapp that I want to stay in touch with then I'm going to download another app. If there's a group chat with this person then everyone in that group is downloading a second app. And then second degree and third degree connections get affected too.
Getting out of the EU would be a massive blow to any established social media company that depends on network effects. It doesn't matter as much for smaller companies that are still growing (eg. early Facebook still managed to grow by only offering services to college students).
