Two mathematicians could literraly communicate encrypted on a piece of paper, and there would be no way to stop them other than scaring them put of doing it with threats of violence, jail or similar.
There is an aspect ro this we have to acknowledge: we live in a world where everybody who knows how can create encrypted communications that are impossible or at least very costly to break. You cannot stop them from doing it.
If we ban that kind of communications, the only persons making use of it will be people who really have something to hide. Criminals, drug cartels, financial fraudsters, terrorists and the likes. Which also means they are the only ones who got to get safe communication channels.
Banning a safe and private way to communicate for billions of people because of some criminals that you cannot deal with is very bad deal to strike. The way I see it criminals most of the time have to interface with the real, physical world in some places and that's were you would catch them if that was your goal: money transactions, buying selling things, shipping, production, etc. Hell if you check any police report ever this is where they catch the people.
That makes you wonder if banning encrypted communications for a whole population is not rather a thing you do for politicial (power) considerations.