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Battery replacement revenue and upgrade motivator.

Other manufacturer's phones do not last long enough to generate any post sales revenue through consumable replacements. Apple designs good hardware and supports phones reasonably long enough to at least generate one post sale revenue replacement (for those who don't already replace their phones frequently to generate another sale). Designing larger and longer lasting batteries will cut into those revenues.

Starting off a brand new phone with just enough battery to get through a work day results in the battery life degrading to requiring at least two charges a day after a couple of years of heavy use. As apparent in these forums, low battery life is a powerful motivator for people to upgrade their phones.

After a couple of years' heavy use? My iPhone 5 needed charging twice a day as soon as iOS7 was installed. It was only a few months old prior to that, and I used to get nine hours of usage time. The moment iOS7 installed I got four or five hours of usage time. The battery was fine, but thanks to iOS7 it will last half as long.
 
. . . If the iPhone was one millimetre thicker you'd probably have an extra 25% battery life and virtually no difference in heft or hand feel . . .

I've been thinking this for a while now. I'm actually happy with the thickness of the iPhone 4S and with the the larger footprint of the iPhone 6 with the 4.7" screen, you could fit a sizeable battery into such a chassis.

That would be a far greater priority as far as I'm concerned.

Just look at the Sony Xperia Z2 for example with a 3200 mAh battery!

Admittedly it has a bigger screen and therefore a larger body but, with a thicker iPhone 6, Apple could have given us all a lot more juice to play with.
 
After a couple of years' heavy use? My iPhone 5 needed charging twice a day as soon as iOS7 was installed. It was only a few months old prior to that, and I used to get nine hours of usage time. The moment iOS7 installed I got four or five hours of usage time. The battery was fine, but thanks to iOS7 it will last half as long.

Again a motivation for you to upgrade. If you and everyone else kept the same phone for years - that would hurt Apple's revenue flows. Every company seeks to maximize revenue, Apple is no exception.
 
Too thin means difficult to hold, tricky to pick up off a flat surface, and easier to bend. Plus we might decapitate the cat if we drop it.
Everyone would prefer a longer battery life and hardly anyone wants a thinner phone. They're already thin and light enough.
 
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