THe elements would be way too big for an iPhone. Just think about it. The iPad mini would have smaller elements than an iPhone 6 yet it would have a larger screen and be held farther from the face. If they make an iPhone 6 with a larger screen, they will have to choose a new resolution and keep the same size elements.
It's not the end of the world. Developers have had to deal with it before, and they were happy to do it because iOS devices are so popular they'd be crazy to fold their arms and refuse to do it. With very few exceptions, an app that was made properly in the first place can be remodeled to any new resolution with relative ease.