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What’s the endgame here? Open-source encrypted messaging software is widely available. The cat is out of the bag. Every country on Earth could outlaw the technology and it would still be trivially easy for criminals to use it."
There is no endgame, strictly. Like a "war on drugs" or terrorism, crime will never raises a white flag and surrender. So, no endgame. For insight on the point, I think it's useful to compare CCP strategy 1 to CCP strategy 2: soft power.
I mean soft power broadly, rather than the strictly geopolitical play. Early on, China sought absolute goals, like the USSR and others. The literally tried to control knowledge itself. Strict bans on books or encyclopedias with a bad narrative. Overseas travel was considered dangerous, and vetting was required.
These days, instruments like the great firewall don't work that way. People can work around the great firewall, travel and access information (eg foreign papers) if they want to. A short wave radio, a foreign magazine or a vpn are no longer serious contraband. But, most of the stuff that most people access is approved. They can't keep news of something big a secret, but they never really could. What they can do is monitor, manage and influence the conversation. High influence is the goal, not strict control.
Same here. Sure, sophisticated criminals can avoid fb. But, most people do use fb. This "most people, most of the time" includes criminals, friends of criminals, etc. This is enough. Enough to be chilling. Enough to make secure systems feel sketchy. Enough to help with the the job, whether the job is investigating a string of robberies or monitoring separatist rebels.