Even with "E2E" encryption with iMessage, you're still trusting Apple completely and totally with key distribution. If a new device is added to your account by an attacker or by Apple themselves, your existing devices will happily loop them in to share iMessage access with them. The vast, vast majority of iMessage users are never going to dig into the Apple keychain app to actually check what keys are being trusted and this is something that can be targeted to a single account so no one outside of Apple never even needs to know it happened.