The iPhone 7 isn't any thinner than the iPhone 6 though.
And I think they painted themselves into a corner with the seeking of such slimness.
I have owned phones from the Sony Xperia Z line for the past for or so years, and they're great. But put them down next to an iPhone 6, you (just about) notice an extra half millimetre or so of thickness.
Just about enough room to put a nice rubber gasket in around the headphone jack, so that the phone is waterproof.
Having looked at them, I think that the iPhone 6/7 are too thin to waterproof AND have a headphone socket. They only had three options - waterproof it and make it thicker, waterproof it and remove the headphone jack, or don't waterproof it.
The wanted waterproofing, so that left them two options. I can't say for certain why they chose to remove the jack, but I suspect that Apple view making the device thicker as being a step backwards, and they lacked the courage to do that and put a bigger battery in. So they took the only way out that remained...
Of course, if they were Samsung/Sony/HTC/Huwai/Whoever, they could have just tested the market with another model. But they're Apple, and want as simple a product lineup as possible - hence my verdict of painting themselves into a corner.