1) You don't believe Apple could enforce rules for app stores that they allowed in iOS? I am quite sure that Apple would not be forced to allow alternate app stores onto iOS without being able to set some standards. 2) Most users would continue to use the Apple App Store just like most users on Android still use the Play store even though there are other app stores. So the developers would not flee or if they did flee it would be to their own peril. On Android there are advantages to buying apps through the Google Play store and I am sure that there would also be advantages to buying apps in the Apple App Store on iOS if there were other stores.One massive problem though: developers will flee Apples app store because it's "cheaper" and pretty soon, my regular iphone is useless because I have no apps to choose from.
Meanwhile, all these developers are creating the worlds biggest unregulated street market of app stores and anarchy ensues. The big players quickly create their own app stores that host only their apps. Epic comes along with their app store and developers-- sick of all the nonsense on the open market-- move to the Epic store. Ten years from now, politicians are calling for the breakup of the Epic store because it's a monopoly on Epic stores.
I'm not really trying to be funny here because all of this could happen, and probably will. The only thing I even remotely support is the ability to run other operating systems, like Android.
I really don't believe that the inclusion of alternate app stores would make much difference to the average user experience on iOS at all.