I obviously have the view that the walled garden benefits consumers, so my bias shows. And, I think there's a lot of wiggle room on the cost to host in the App store.

However, US monopoly law is based on consumer harm, so...

I think on a mobile platform, with plenty of competition to not buy an iOS-based phone, a walled garden absolutely benefits consumers more than it hurts. There is so much PII on phones now. Given the total lack of InfoSec knowledge, especially at the mobile-user level, a walled garden is crucial: see every Google Store vuln that hasn't hit Apple.