Encryption is not a secret. It's accessible to criminals, and criminals don't give a shit about "backdoor" laws.
In fact, I'd venture to guess that there is great encryption software already available on jail broken iPhones.
What we have also seen in regards to just use of technology is the reign of default. I I doubt that criminals would go for an unlocked iPhone for security reasons for a few reason but one being that is beyond them.
The bits I've read suggest that they don't - or at least not widely.
Eg: https://www.schneier.com/blog/archives/2015/11/paris_terrori...
The rest of the speculation in that article was called out and refuted by the link I supplied.
I apologize, if I'm wrong, but I can't help but feel like you're being disingenuous.
1. http://motherboard.vice.com/read/isis-now-has-a-propaganda-s...