Your point? Most customers in the marketplace are averse to change across any service. It’s not uncommon for users to stay with single providers due to momentum.
My point is that saying iPhone users are by default AT&T users rests on the assumption that people have stuck with the same decisions they made about mobile network and phone operating system that they made over a decade ago. That isn't even factoring in the growth of the market overall and the people who have bought their first smartphone within the last decade.
The churn rate for wireless carriers is around 2% per year in the US, give or take. There are about 300M wireless subscribers in the US. Meaning that around 6M wireless subscribers per year switch carriers.