This is all fine if other stores existed on the phone that could compete with Apple's offering. If it's such good value then developers would choose the 30% cut

This is all fine if other stores existed on the phone that could compete with Apple's offering. If it's such good value then developers would choose the 30% cut over publishing on an alternative store with less fees (and potentially less benefits). But as Apple holds a monopoly on app stores on the iPhone this assumption that the cut justifies the value offered can never be tested in a real market setting.