Having a system where trust is embodied in a single entity is one valid solution. It's also not the only solution and I haven't heard anyone claim that it is.
That is technically a remark I agree with, but you're skipping past the actual point of my comment: it may be a valid strategy on its face but it is fragile and makes users vulnerable to systemic exploitation.
The web browser ecosystem has its own (different) problems, but iMessage lacks requisite variety to back up its particular claims to privacy and security (see that Reuters article for a preview).
I skipped past that because that wasn't what I had expressed disagreement about. Though now you elucidate further I'll say I fundamentally disagree with your "actual point" as expressed. While I agree that systems of distributed trust are fundamentally healthier, they are an order of magnitude harder, and rely upon educating users. And some percentage of users will always be impervious to education — see the continued prevalence of phishing scams for example.
A system which relies upon trusting fewer entities is inherently less fragile and less vulnerable to exploitation. It's true that systems can be designed which rely on users trusting a large number of entities, and can sometimes result in a more educated user base, but they're much harder to implement and much, much, much, much rarer in the real world.
I think the difference here is whether we're considering the plausibility that there aren't any security violations versus the overall frequency and severity. Centralization significantly increases the chance that all the systems involved will be safe; that's what makes it so useful for individual organizations, where centralizing their operations wouldn't attract significantly more bad actors to try breaking their security than decentralizing.
But if we have centralization on the scale of a society, then anyone interested in any of the groups using that centralized source of secure data storage/transfer will be drawn to look for the flaws in that source. And there are always flaws, either technical, legal (as with the government spying mentioned elsewhere in the comments), or otherwise. And once any group manages to infiltrate that one source, they get access to everything dependent on it.
Sure, decentralized security is harder to get together, meaning we have an initially-high violation rate that decreases over time (though this can be supplemented by security-conscious users taking their own steps to protect their data).
But centralized security at sufficiently large scales essentially guarantees a breach impacting everyone within its domain; and the kind of trust that would be required to sustain such centralization also anti-correlates with users independently adding additional layers of security to their systems.
This seems like a much greater risk than just accepting that users who are "impervious to education" will be vulnerable to certain social-side exploits, while everyone else will be reasonably safe.
The web browser ecosystem has its own (different) problems, but iMessage lacks requisite variety to back up its particular claims to privacy and security (see that Reuters article for a preview).