It’s a decision we’ve already made. Actually, we cannot even change the version requirement after the initial version has been published in the App Store.
As you probably know, iOS 7 dropped support for some older devices with weaker performance. Making the minimum requirement iOS 7 allows us to better optimize the game because of a smaller set of supported devices. We can push the game further technically without having to make compromises for older hardware. We believe this will pay off in the years to come!