There is ample evidence that breaking or backdooring apps or devices (more or less specifically) targeting criminals, allows the cops to roll up huge crime networks. These are expensive and time consuming one off operations.

Main stream messaging apps like Signal and Whatsapp of course rely one very similar technology so it's probably safe to assume some illegitimate stuff is happening there. I wager that finding those criminal messages in practice has a more serious issue beyond encryption: there is far too much data.

And even if encryption were banned altogether, the criminals would surely find a way to whispers their mischiefs in stenography.

*If the criminals would stop using encryption now that it's banned. Alas, criminals sometimes break laws

The debate is whether it is worth it or not, banning encryption leaves the nation more vulnerable to authoritarian states, and even leaves its citizens communications more open to criminals because if they hack the state department that has the keys, they get access to all the comms