Why the focus on point-to-point communications?
So Alice and Bob (and their circle) develop their own cipher. Suppose that the cipher is a book cipher. Any message sent will be encrypted twice -- once in their private cipher, and once in some backdoored public cipher. How does the backdoor help the government (or anyone else who is listening)? The metadata in this case says Bob is messaging Alice...but so what? And in the case that Bob simply posts the message on The Register -- then the recipient(s) are likely completely unknown!!
For example, here's a (real) book cipher message. What does it say?
sforzato pharyngo- woadman mecometer semihysterical veratrize fiercenesses Ranquel lepidotic Kawaguchi eyeservice fringiness half-plane piligerous saskatoon straddle-fashion sharecroppers colibertus bilobular unsacrilegiousness Gallicolae snake-eyed hydrophorous rain-soaked entoplasm eschewing brulyiement Erastianize acetphenetid recheat hout alada superaffiuence sweet-scented Altingiaceae researchful unegregiously unregenerately blighted Marlette nonbeauties Ossetian perversite artcraft Staley physiognomonic keawe kentallenite acroataxia yodles Rhabdomonas mournfulness VC loose-lived self-purifying tornadoesque uroo slopmaking annalists undeferrable ammonitic WAN pokable limbs Composaline gasified Chibcha elephantiases guerdonless orchestras whoop-de-doo commercialised periclean half-reclined naturata haemonchosis bug-juice theorically demonstrant premarrying honduras knickknack Adrianople -aceous inductees counter-faller cervicorn yowe adenomata kutch jardon eradicable nonfervidly cribriformity totoaba Marduk Muscadine mangrate Californian Mignonette Stroessner fisherpeople So. gibble-gabble cayuses Wallinga squab-pie fancywork niftiness