Do we have secure SMS messages? Do we have unified mobile messaging with a mature protocol that is both widely accessible as well as secure? How many more decades shall we wait? (asked rhetorically, yet politely) Yes, I argue it has failed, and an SMS/MMS successor would look like a descendent of iMessages and Google's RCS.
iMessages supports CALEA (but is still end to end encrypted, as well as FaceTime), all US messaging products must support it (even Twilio supports it [1] [2]). You are unable to subvert nation state legal jurisdictions within fundamental telcom infrastructure. Signal can get away with it, Apple, Google, and cellular carriers cannot.
Can't have your cake (universal messaging) and eat it (end to end encryption with zero trust requirement) too. So can't we push from something better than SMS, using regulation if required? It must not be perfect, but simply extensible in the future (similar to SS7). In jurisdictions that are favorable to it, you support E2E encryption. In those that are not, you downgrade loudly. Compromises must occasionally be made.
[1] https://www.twilio.com/legal/law-enforcement-guidelines
[2] https://support.twilio.com/hc/en-us/articles/223136547-Submi...