While sales for the iPhone 5S were incredibly strong, most would agree this was despite the phone's screen size, not because of it. Now the phone is bigger, there's been an explosion of upgrades. Even if Apple do return to the one-handed-use phone market this year or next, plenty of people prefer more space for content over reachability.
The Verge's report: Apple knows customers are used to phones with screens larger than 4 inches:
http://www.theverge.com/2014/4/4/5571926/apple-document-shows-concern-for-iphone-sales-decline-need-for-bigger-iPhone
"Our data clearly shows that bigger is better for most consumers," the report reads.
http://www.cnet.com/news/the-world-wants-a-bigger-iphone-and-big-smartphones-in-general-surveys-say/
Even in the US - where interest in 'phablets' was relatively low, 40% of people wanted a bigger phone:
http://www.informationweek.com/mobile/mobile-devices/smartphone-buyers-want-larger-screens/d/d-id/1278766
Yahoo! Report:
https://www.yahoo.com/tech/survey-says-people-want-bigger-phones-89679157394.html
T-Mobile says 77% of its customers want a large screened phone:
http://www.slashgear.com/t-mobile-says-77-of-consumers-want-a-large-screen-phone-17238969/
'Go big or go home': http://www.technewsworld.com/story/75671.html
If you want more, reply to this comment.
I could list a bunch of articles in which the author was wrong. That's not proof of anything. Sales can't be argued with, and people were buying smaller devices in droves.