If it were that simple, wouldn't they have more than 20% market share? It seems clear to me that iPhone owners are paying a premium for the branding, which is pure profit from Apple's perspective.
"People pay premium for a premium product" doesn't signify to me a subjective difference. Especially when it's used to argue that the premium paid has nothing to do with Apple's branding, an objective and extremely common reason that people pay premium prices for products that are objectively identical to their competitors.
This is not a rhetorical question: I could imagine a study that would explore the influence of brand vs. size, density, materials, etc.
Also, since when did rationality enter the discussion? A beloved wants diamonds so their partner buys them. But diamonds are not worth anywhere near what's commonly paid for them.
There is a far mix of rational/irrational behavior in most luxury purchases--it's not an Apple exclusive trait.
Want an iPhone, you'll buy THE iPhone for whatever Apple wants to charge.
Want an Android phone, you'll select from a zillion basically identical competitors all more or less profit free (because competitor A can't extract an extra 5% profit if that would drive 100% of competitor A's sales to sellers B thru Z)
This shows the fallacy of "the smartphone market", there's the non-competitive iphone market with about 20% sales and the hypercompetitive confuseopoly market the other 80% buy into.