If iOS9 adoption is fast, developers can start building iOS 9 apps that include 3D Touch features. The 3D Touch support will obviously only work on the 6S serie

If iOS9 adoption is fast, developers can start building iOS 9 apps that include 3D Touch features. The 3D Touch support will obviously only work on the 6S series for now, and there may be different code paths involved but it's easily do-able. If a majority of users stuck with iOS8 for some reason, developers would need to target that, iOS8 apps couldn't support Touch 3D features even optionally since none of the relevant APIs would be available.