Regarding the "Apple supply bandwidth, infrastructure and payment method" argument that crops up commonly in these discussions - its a facetious argument because the seller HAS NO FUCKING CHOICE IN THE MATTER even if they do have the ability to do it.
I'm sure both Adobe and Microsoft have the ability to replace Apples "contribution" completely, but they aren't allowed to by Apple. They aren't even allowed to attempt to, its completely verboten - you have to use Apples distribution service, you have to use their payment gateway, you have to use their app store otherwise your app simply won't happen.
And thats the point being made. Its not about what Apple offer, its about them refusing to allow those than can equal them in capability to actually do it.