At this point I think Apple feels like there are enough apps in the app store so they don't have an incentive to drive more participation. If anything, the fixed cost of vetting an app should be the same for the big guy and the little guy and the big guy can spread that over more revenue so Apple could afford to give a break to the big guys, not the little guys.
It's actually not a duopoly, but two monopolies:
If you're on iOS, you have to eat the Apple App Store cut.
If you're on Android, you have to eat the Google Play Store cut.
They don't actually compete with each other in any meaningful way, so there's not even price competition between them. As long as they're able to control their respective platforms and ensure they're the only successful app stores on them, they can charge whatever they want, and have no incentive to lower their rates.