I've got bad news for you: that is happening, to some extent, already. It's part of how they caught Ulbricht. They find it far more practical to use collected

I've got bad news for you: that is happening, to some extent, already. It's part of how they caught Ulbricht. They find it far more practical to use collected metadata from addresses to identify postal traffic flows, however, and target their interception based on that.