As for making illegal any form of messaging app not control or regulated, that is up to each government, those that are openly totalitarian will most probably try to enforce it, which is almost impossible, but they can legislate and make the life of those using it difficult and even incarcerate them if caught... EU is going that way for the surprise of many, USA is not much different, what can't control with back doors normally is legally banned and their devs incarcerated (tornado, Samourai wallet, etc)... 

 Yes, I was surprised that Durov is accused, among other things, of using encryption in his application without a license... 

 I think that one, is a can of worms at a complete different level...