Until possession and distribution of such apps becomes a crime in itself.
That's the only way to enforce it and there is history for it in cryptography export laws and anti-circumvention laws.
Criminals don't care about what is "legal", that's what makes the criminals in the first place and in any case, what are you going to do? Ban encryption entirely?
Perhaps we also should ban mathematics and books while we are at it? After all, criminals can read chemistry books and learn how to make explosives..
Apparently the cartels in Mexico have discovered a way to mix crystal meth with gasoline, put it in a cars tank, drive it across the boarder then separate it from the gas for distribution. The process is supposedly currently unknown to science (I’m no chemist so can’t be sure).
Think about it - if you're a criminal, and you know about chat control, why would you risk your chats being leaked at all? Why wouldn't you use a different app that you know to be more secure (this already happens for any serious crime already btw)
It's precisely the law-abiding people whose privacy will be invaded for no-gain