And since Apple owns the infrastructure and we can't validate anything about it, there is little reason to believe they couldn't maliciously skip steps.
Think of an SSH client that never verifies host keys and never warns you when they change. That's basically what iMessage is.
At least with iMessage, the entire ecosystem is managed by one company, so you only have to trust one company.
It does eliminate the requirement to trust Apple actually. That's the point of (real) end to end encryption. With a custom client, you could use them as a conduit to deliver your message without using any Apple software on either client if you wanted to. As long as the users can verify the keys.
> At least with iMessage, the entire ecosystem is managed by one company, so you only have to trust one company.
I just explained at least 3 different reasons why that is not true. You can fully "trust" Apple and still compromise an iMessage conversation.
> It does eliminate the requirement to trust Apple actually.
As the parent comment indicated (emphasis added), in an extreme hypothetical scenario, Apple could (be compelled to) write a backdoor specifically for your device that captures all text and touch input events, or periodically takes screenshots and sends them somewhere.
This is like sending a PGP message over gmail using mutt on linux via IMAP. Google owns gmail, but they can't backdoor you or read your message in this scenario. Any non-broken secure messaging protocol should have this property, regardless of if people use it that way. This is what end-to-end encryption means.
Okay then, tell me how you could build a system that
* Guarantees secure transmission of public keys
* Can be installed safely on a closed-source operating system
* Is impervious to the whims of the hardware vendor or the operating system vendor
* Is sufficiently straightforward that mums everywhere will happily use it
* Doesn't require users to become security experts
Given the practical realities Apple has to contend with, I think they've done a pretty good job -- certainly a lot better than what anyone could have expected, arguably a lot better than anything comparable that has come before it.
This discussion about backdooring applications isn't even interesting or relevant here. The protocol itself is not secure.