You call two-and-a-half years mid cycle? You call the iPad Pro mid cycle?
It's indisputable that the majority of 4" iPhone owners will not change to a bigger model.
Apple tried two series of big phones and finally conceded that they had to continue with the 4" line, just as they had to release a stylus and keyboard cover.
Now that the 4" line has been updated, we will see people coming back to it while some will also try the bigger phone. One would expect the bigger phones to dominate sales because of the popularity of Android, the needs of Asian users and the number of models and accessories, plus some additional features.