Right now if you use encryption the authorities have no proof you’re doing something illegal, because you might not be. But if they make (secure) encryption itself illegal, then anyone they aren’t sure about suddenly becomes a criminal they’re sure about. Then it’s just a matter of selectively prosecuting those whom they most dislike. So it doesn’t matter to them that much whether lots of people find a technical workaround. If they can’t read your messages that’s all they need to be able to silence you if you’re inconvenient.