One problem with that is that Apple’s restrictions allow them to eliminate piracy, and perform an approval process that keeps high the perception of iOS app quality. These two things make the iOS app market much more attractive to developers. Allowing Android-style competition would lower the value. It might be that it lowers it by less than the difference between 30% and a “fair” cut, but it’s hard to tell and users and developers wouldn’t be well-served by performing an uncontrolled experiment to test the theory.