This could be true... but only if there is an actual specific problem that they can put their finger on that requires the device to be AI-first. What is that problem exactly?

It's also not obvious to me that a concerted effort by Apple (unlike what we've seen so far, admittedly) wouldn't eventually be successful in converting the iPhone to something effectively indistinguishable from a platform designed from the ground up to be "AI-first".

Designing things from the ground up is hard by the way. It's not just the design itself; it's the ecosystems around them which are really hard to get going. Apple has the world's biggest flywheel in motion there already.