We're not talking about the world. We're talking about the universe of the iPhone.

In the brick-and-mortar world, malls are fungible, and stores that sell the same goods are fungible. The iPhone is not fungible.

We are talking about the universe of mobile devices.

You are; I don't think we are. Spotify certainly isn't, and it may be entirely reasonable to view the world through that lens.

In relative numbers yes, but I wouldn't call short of a billion active devices "quite small".

Still, there is a choice for anyone that wants to sell products.

Anyone that wants to constrain themselves to a single market, needs to accept the rules of focusing on a single market.