8MP is the sweet spot for any camera on a phone, upping the pixels all of the time, just means smaller pixels & more noise in the long run.
If the iPhone 6 still had a 8MP camera with a larger sensor and larger pixels & faster optics ie F1.8 or F2.0 that in itself would be a big improvement over the 5S, and would easily put the camera only second to the current Nokia 1020, or even give it a run for it's money.