How is this any different from how things already work? Even if you use iMessage today, there's no guarantee that the person you're messaging isn't piping your conversation into an LLM tool or using Beeper or similar to divert it to another device. And that's not to mention if it falls back to SMS, or you use a different messaging platform that just isn't set as a default.
If your friend violates your privacy rights (either intentionally or unintentionally), there is both criminal and civil legal recourse. That is different in kind from legally requiring people not be allowed to prevent violation their privacy through reasonable safeguards. But, hey, if you truly believe your argument, you’ll take the locks off your doors since people can break in anyway. Why have such a reasonable safeguard against physical property rights if you won’t allow reasonable safeguards against digital property rights?