I'm confused at the assumption that you can prevent serious organized criminals from having access to strong cryptography simply by backdooring common communications apps.
The genie is out of the bottle: Powerful criminal enterprises will have no difficulty hiring people to build overlay tools that they can run inside their backdoored comms that will provide adequate (at least, if not effectively unbreakable) cryptography.
Even if we ignore the considerable first amendment barriers and there were an effort to outright outlaw strong crypto, steganography has become very strong and even if not quite sufficient finding out your comms were being monitored via a use of crypto charge is way better than having the attacker learn all your info.
In light of this, I find it difficult to see the goverment's position as seeking to widen infrastructure for the wholesale surveillance of the general public with claims of terrorist groups as a dishonest cover for the demands.
The old adage "If guns are outlawed, then only outlaws will have guns"-- has serious limits owing to the nature of guns as physical objects. The sentiment applies a million fold for cryptography, which is fundamentally a collection of ideas with a zero marginal cost of reproduction or execution, mass-less, volume-less, capable of being distributed around the world in a fraction of a second and implausible to silence even with great leap forward mass murder.
Actions to block access to encryption would severely degrade the security and privacy of the general public, on this point I agree with the author. But would it prevent pedophiles and terrorist from having access to encryption? Not likely.
Would pedophiles and terrorists occasionally screw up and use insecure means... sure, but they already do that today.