Until we see another paradigm shift in terms of smartphone technology, I think we've reached the apex for screen sizes at around 5". In 2007, Apple set the standard with the 3.5" display. But by 2011, flagship Android phones were 4.3" and getting larger. Today, it seems most high-end phones are anywhere from 4.7" to 5.0".
Phablets are here to stay (6 Plus user myself), but based on the people I know, and all the threads on MR about returning the 6 Plus, they are still a niche product. Crucially, people want phones that they can still operate to a certain degree with a single hand, and can put in a pants pocket. Perhaps making bezels thinner and having curved displays will help, but for now 5.5" seems to be the absolute peak size when it comes to the average consumer.