The camera sensor is a commodity, lenses are limited by physics. UI has basically converged/standardized. The A series is 25% faster than the competition. Apple's marketing is great, but Samsung and Xiaomi are no slouch. Peer pressure alone would assume hordes of brain dead consumers, which is unlikely.

What is the other major differentiation the iPhone has to justify being among 7 of the 10 most sold phones?

You may choose to only make assertions given Tim Cook’s spreadsheets, or you can choose to infer based on the data we have. You can also skip this thread if you think that’s just idle speculation, of course.