except when the app leaves the App Store.
immediately your whole post goes up in flames.
Here's reality...
The whole iDevice planet is 16+ year programmed to get all apps through the Apple App Store. Anyone voluntarily pulling any apps out of it- and I acknowledge some certainly will try- will see the plunge in established revenue flow from NOT being where most shoppers shop. The sudden cessation of that revenue will motivate them to get right back in there again. I doubt that many will choose to sacrifice that easy, established
money... even if they know they can make more per app unit sold by selling direct or in other stores with lower seller commissions. The smart thing for most to do is both stay
AND go.
Now, I also expect many to offer their apps direct and/or through collaborations with other independents in shared stores, etc too. I think some who are long-term frustrated with Apple taking the first sizable bite out of each sale will even try to go exclusive but then the termination of easy, established money will likely bring most of them right back again... ASAP.
Net result: most apps will still be for sale in the Apple App Store too. Does this make sense? How is it in the Mac App Store? Apps are for sale there, cutting Apple in big on those transactions and some of the same apps are available direct (and/or in bundle offers, and/or still on retailer shelves or digital downloads), etc. So obviously, that works.
However, let's put my "flames" towards your take and fan them by imagining some app sellers want OUT of Apples store so badly, they'll just forego that easy money to go their own way. If EU people want those apps, they would then be pressed to buy direct- exactly as that situation works with some Mac Apps not in the Mac App Store but available direct or in other stores.
Let's further push your apparent point that some customers won't want to buy elsewhere so what will they do? There are MILLIONS of apps in the App Store. Pick almost any given app and you can find a bunch of similar ones with similar capabilities. If someone yanks a desirable app from the store and EU customers want those functions, they will likely just adopt
another app with those features.
And if some features are somewhat unique to some app and that creator yanks the app, some other app developer will seize upon the opportunity to work those unique features into their app so that "App Store of Bust" customers can buy the same FABs but in a
different app, still in the Apple App Store.
Along with previously-offered, very-tangible benefits of competition, ANOTHER BIG ONE is that it makes product creators more aggressive in competing on FABs too. In other words, it's not only PRICE competition, but also VALUE competition. So I basically dare many apps below the very big dogs with deep marketing budgets to yank their apps with a "shoot themselves in the easy revenue foot" stubbornness to stay OUT... and then watch other app developers replicate their FABs in alternative apps that remain for sale in the App Store too.