Today, if you want to securely plan some crime, you set up PGP or whatever and use cryptography. In years past, you might have used a pay phone, or mailed a letter to a neighbor or relative. You could come up with a relatively simple code based on some book you both owned that would be effectively impossible to crack. There were lots of things you could do to avoid spying, if you wanted to.

Historically the government has not been able to automatically monitor everybody's communications if they wanted to, with just resource restrictions preventing it from happening en masse. Historically, the government has always been able to monitor communications only if the targets didn't take adequate precautions. What's changing is that the necessary precautions are simultaneously becoming much more difficult (planning your crimes on a pay phone won't save you anymore) and much easier (the crazy math needed to protect yourself is largely automated).

Really, though, I think the police should suck it up and work with it. Used to be if you wanted to tap someone's phone you'd actually go out and mess with their wires to add a physical tap. The same idea still works! Get a warrant and add a keylogger to their PC, or a microphone to their car, or whatever you need to get the info. Historically, spying on people always took legwork.

I think the US mail is more secure, and it's been delivered by the government since way before computers existed. Sure they now scan the meta-data, but the content goes undisturbed except for specific interventions.