The question is how to enforce it, of course
Sure, most people will stick with Facebooks WhatsApp. But we should assume that those aren't the people the government is interested in.
The ones who really want to hide something can just download an app developed in a country that doesn't impose blanket surveillance. Sure, using an app like that can be made illegal, but note that these are people that likely don't care much about this.
In the end, the only one being spied on are the law-abiding citizens. One wonders if that was the plan all along.