I'm definitely not advocating illegal activity but I do wish to state some facts.

Do you know how much data traverses the Internet every second? It's an insane amount and finding criminals that you have listed above would be like finding needles in haystacks, especially since the intel community are apparently well understaffed.

Then you add some complexity like VPNs where the headquaters are in non-extradition countries like Panama who are basically going to laugh at any Government if they try to enforce backdoors. It only takes maybe an hour or 2 of Google-Fu for criminals to learn how to evade watchful eyes.

So to answer your question, I think we should leave encryption alone instead of trying to weaken it (which I stated above will never be 100% anyway and will only catch low level dumbasses). I don't have the answer or solution but perhaps more funding and manpower needs to be given to law enforcement to carry out operations like social engineering.