Precisely. Stores aren't just competing to get users, they're competing to get developers. If one store offers a higher profit margin for developers then that store will have the best apps.
If someone could launch a competitor to the Apple App store, and say "I'm going to take half the margin that apple takes from app developers", I think developers would flock to that store, and then so would customers.
If someone could launch a competitor to the Apple App store, and say "I'm going to take half the margin that apple takes from app developers", I think developers would flock to that store, and then so would customers.