Hmm...this is an interesting idea. Nothing says iPhones can only be launched in September.
There are two trends to evaluate:
(1) Apple has only ever launched iPhones in September/October; it has never done so in another month.
(2) Apple has always taken the current top model and moved it down to the lower tier model, dropping the previous lower tier off the chart.
(3) Apple has, since the 3g, offered new designs only once every two years.
Is Apple likely do diverge from any of these? Would they diverge from one but not the others?
I'm disappointed if they don't release a successor to the 5c. I would like a 4" phone, but more important to me, I like the materials and styling of the 5c better than I liked the 5/5S and the 6/6 Plus, considerably. If they came out with a 6c that wasn't 4" but was instead a 4.7" phone with the plastic case akin to the 5c, I'd be perfectly happy.
Assuming they do not come out with a 6c, and that they do not change the designs, I may in fact opt to ride it out with my 5c another year. I'd be missing TouchID for another year, and Apple Pay...but perhaps I will like the offerings better a year from now. It appears that iOS 9 is intended to run fine on my 5c, so I'm not worried at this point about that.