I wouldn't call them fairly different.
They certainly aren't from a technical view. The app distribution stores work very similarly, and the principle of apps being code-signed are virtually the same. And you can "sideload" on iOS just as you can on Android. There are even
alternative App Stores that exploit Enterprise developer certificates to do so. They can also do so to distribute spyware.
The difference?
On Android, you can selectively allow apps from other sources, after having been warned. You have choice.
On iOS, Apple just restricts the number and selection of developers for which you can.
It'd only require a very minor change in Apple's policy - and
no change from a technical standpoint - to allow "sideloading".
The choice will remain.
Everyone's to obtain his apps from Apple's walled garden exclusively.
Will there be some developers that leave the App Store?
Yes, some (though if Android phones are any indication, it won't be too many!).
For these apps / developers, the "choice" would then shifted towards choosing a different app or sideload.
I'd argue that's much less restriction of choice than only being able to buy from one store that has all the apps.
👉 It is then up to Apple to remain competitive by offering an attractive platform at competitive commission pricing for developers and competitive prices and convenience for consumers.
I hate strawberry ice cream but love chocolate ice cream.
I also just a ice cream a few hours ago today.
Thankfully, entry barriers to the ice cream market are low and there's plenty of choice and variety.
I mean, if a duopoly of two American trillion-dollar multinationals...
1) controlled 97+ percent of the market for ice cream sales in my country, and also
2) controlled the market for ice cream machines / makers, and the biggest one of them
3) required all independent artisans (developers) of ice cream flavours to distribute their produce exclusively through them
4) and then prohibited certain (legal) flavours of ice cream from being offered / distributed cause they don't like them
I'd be all for EU/government regulation to promote more choice, variety and competition.