I do not understand this approach. A market solution works and is popular, so the government should step in and try to develop something better with "contracts"?
(It also failed; as of 2014, .doc was still used more than .odt by orders of magnitude in the major institutions - Commission, Council and Parliament)
If for some reason WhatsApp suddenly vanished, people would switch to the competitors, like Telegram, Messenger, Viber, etc.
There would be chaos during the change, but that's just because the change happens suddenly. If it happens slowly (if the government continues to block WhatsApp, people will rely on an alternative), then I see no harm.