According to my rather brief examination of iMessage, you can't verify key fingerprints in iMessage. It means that Apple can install MitM on a probed subject any second, and you would never know that it is there. See:

You <-- end-to-end-encryption --> Apple MitM <-- end-to-end-encryption --> Your buddy

You have _zero_ control over it, and the only thing keeping your secure and private is Apple's pinky promise.

> The same is true for any app running on iOS/Android.

Umm, no. There are such things as open source, verifiable builds, and, yes, decentralized messaging protocols. You can take an open source client (for example, from a reputable source like F-droid), and connect to a server totally unrelated to client developer. You can run an encryption protocol where you can actually exchange public keys, verify your keys fingerprints, and confirm the identity of your chat partner. That's what people really concerned with privacy and security do. Others are satisfied with a promise that sounds good enough.