Quite. The depth of a mobile phone is dictated by the dimensions of a battery that's sufficient to power that phone for an acceptable length of time. Unless Apple have uncovered some magic battery technology that enables them to shave several millimetres off its depth, the iPhone is going to remain as 'fat' as ever.
If super-skinny batteries are feasible at a realistic price then I'm sure there are people who would be willing to pay a $100+ premium for a skinnier phone. What absolutely no-one will want is a skinny phone that's only been made possible by halving the battery life.
With the iPhone 5 newer LTE radios that used a quarter of the power of previous radios came along. That couldn't happen again? Just pointing out that the other way to develop "magic battery technology" is simply to use less power.
I do often wonder what people whose iPhone 5/5S batteries die in less than a day are doing with their phones. I'm pulling off about 2 days right now with a 5S and my other phones don't do any better.