If you define your market small enough anything can be a monopoly. I bet Apple has a 95% market share on phones that cost more than $1500. I bet Google has 75% market share on phones less than $100. So what?

We are talking about the App Store here, correct?

Do you think that for an App Store attached to a phone whether the phone cost $100 or $1,000 matters?

Because everyone related to those App Stores does.

Apple's control of the app store makes the $1,000 phone more appealing. If they opened up the iPhone to other random app makers, the phone would be less appealing and they wouldn't sell as much.