Really good point. I'm amazed we don't hear this argument more, and also the related one that we always used to have an anonymous payment system (cash) so why is a digital equivalent unthinkable?
In the inter-war years and through WW2 there was a GPO Special Investigations Unit in every sorting office.
They were steaming open, photographing mail, then sending them on, and the photos to MI5. They were also responsible for recording phone calls so had presence in major exchanges.