> Before WhatsApp criminals communicated using different means which were easier to monitor

Perhaps sloppy criminals did so, but the careful were engaging in schemes more like what we see on "The Wire". Meeting in secluded, noisy locations wherein privacy was all but guaranteed.

People, including not-convicted criminals, have the right to privacy. If the NSA and law enforcement are going to make it harder and harder to find private places through dragnet collection, it should not be surprising that the populace will seek to reclaim privacy.

If it's an arms race, as you say, it's an arms race that law enforcement was winning at so one-sidedly that the relationship was becoming abusive. Tools like WhatsApp, Telegram, Signal, etc., are the citizenry's first efforts at serious competition.

There is no obligation on the part of criminals to make it easy for law enforcement to catch them, and criminals will use whatever tools they can to stay out of jail. Yes, that's an arms race, but that arms race has nothing to do with whatsapp. That's just a general purpose tool that has been re-purposed. This is what I was getting at with my recent comment that knowledge (and by extension technology) is dual use. You can't make something that will not somehow benefit the bad guys too if it benefits the good guys.