The OP makes a valid point, which is that Apple does fear that the mobile space ends up being driven by a handful of third-party apps, rather than by the device/OS (and the AppStore is part of the OS).
So let me be clear. What is a killer app? A killer app is something that drives you to buy a device. The web browser is a killer app. Office was a killer app. Today there is no killer app IN the Apple App Store. The App Store is the killer app. And Apple controls it.
What Apple doesn't want is for Adobe or MS or Google or StartupX to create the killer app on the iPhone as they could diminish the importance of the iOS platform (since presumably this app will also shortly become available on other platforms).