Does apple having designed, developed, and manufactured the phone as well as having built, maintained, and serviced the App store mean nothing?
The App store is a highly trusted place to download things on your phone, and that's a value that apple provides and that costs money to maintain. Pretending that it's as isolated as you pretend feels very disingenuous.
I'm an Android developer myself, but the app store is commonly seen by my iOS colleagues just as a stupid hurdle they must clear to get their app out to the world. Not as the godsend that Apple portrays it.
If the app store is truly as immensely useful as Apple wants everyone to believe, why not enable full-on Android-style sideloading on iOS and let the app store compete with that on its own merits? Surely everyone would still prefer it if it's so great?