There's no guarantee, but some apps intended for security actually make at least a minimal effort to be excluded from plaintext backups, rather than intentionally sending their encryption keys to the backup service that just happens to be run by the same company...
Ok. So you concede that there is no way for you to ensure that messages you send me, that I can decrypt, are left unreadable by anyone but me.
So what secure communication system should we be using given that none of them can guarantee that the recipient doesn't leak information to another country by choosing to use a compromised version of the client?
My complaint is not about guarantees, it's about defaults. Default non-e2e-encrypted backups of message encryption keys are the problem here. No system can guarantee absolute security, but that doesn't mean they're all equivalently bad. Some are definitely more secure than others, and defaults have a lot to do with it!
You are attacking a straw man. The risk is the your correspondent does not have ADP enabled, as it is not on by default, and not even offered in some authoritarian countries like the U.K., so even without their cooperation they can still get their key. I don’t know if iMessage implements Perfect Forward Secrecy, but at the very least they will be able to read all your messages moving forward.
