Before encrypted electronic communications, criminals and terrorists had to use things like in-person meetings or unsecure communications methods (like analog telephony) to communicate. These were obviously vulnerable to being listened to for a determined party, but that was simply how it was, there was no other option. Law enforcement could use various human-powered means to target specific individuals or organizations, like tapping a particular phone line and having a human listen to it when it went active, or maybe stake out a particular meeting place with some high-power microphones. For the general non-criminal population, while it was technically possible for the government to listen to everyone all the time, it was realistically impractical because of the vast amount of manpower it would require.
Today we're in the opposite situation. Law enforcement can now get ahold of all electronic communications through various taps, but if criminals and terrorists use the proper technology and best practices, it is *impossible* for law enforcement to know what is being said. (Yes, deep-cover operatives are still possible but are impractical for all but the absolute highest-priority things for reasons of time, risk, and the same old manpower problem).
I don't have a great answer. Anything is either too insecure or seems too vulnerable to corruption. The only thing I've come up with is third-party escrow of encryption keys, but who is the third party and how do we know they aren't corrupt?