That may be the case in the US, but in most other major markets, the iPhone is no longer exclusive to one carrier. The entire US population makes up at most 10%

That may be the case in the US, but in most other major markets, the iPhone is no longer exclusive to one carrier. The entire US population makes up at most 10% of the global market. Subtract AT&T itself, and you are left with about 6%. Hardly significant.