On the Mac I avoid the store if it’s possible to download directly from the developer so that they don’t have to pay a cut. If there were an alternative Mac App Store I would avoid it too like the plague.
I'd can see myself using other stores on iOS depending on the store. If Steam existed on iOS I'd certainly consider it, especially if buying a game that ran across platforms included the iOS version.
Although they're more flexible, if a game sells you a CD-key outside the store (eg on Humble, through promotions, etc) they don't take a cut. Steam only gets it when it's through the Steam store.
There is an endless stream of Android and iOS devs saying their app got dropped without warning and the support person refuses to explain or respond to them. That's just unacceptable when an app being removed has the ability to break a business.
This is not unreasonable. Apps like netfix and spotify can not possibly pay the Apple cut and stay competitive with apples own services which pay nothing.
Not a single user is going to pay 30% extra to listen to the same music.
They aren't allowed to tell you how to subscribe, they can just say you must and it can't be done in the app. While on Apples own services, it just uses your already filled payment details.
I should be able to install free apps on my device without giving up my name, address, email, phone number, and device serial number. Apple's App Store requires all of these.