The iPad didn't just do it last year. Ever hear of the iPad with retina display, or the new iPad? It's 100% about marketing and what Apple believes will be the best marketing for the device. I'll let you guess why the iPad Air is called the iPad Air. Just kidding I'll tell you: they wanted to market the fact that it was much lighter and physically smaller in every dimension.
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I think it's a great idea to alienate everyone that wants the 4.7" by making the 5.5" better specs. Oh wait.
All I am saying is it is going to sound sillier and sillier when the number gets higher iPhone 7 iPhone 8 iPhone 9 iPhone 10?
Look at all the other product lines
iPad Mini
iPad Air
iPad Pro (rumored)
Apple is all about simplicity, so if you call that their marketing strategy then I suppose it is "100% Marketing"
Macs did it, iPods did it, iPads did it. Now leaves iPhone.