[DigitalJack]

I wouldn't buy a thicker one. Well, I might, but I wouldn't be as happy. By the time I put a case on my 6s+, it's as thick as I care to go.

There are plenty of cases that have a battery at the expense of thickness. Why not use that?

[khedoros]

I'd buy a thicker phone, but not one with a screen larger than 5 inches. 5" without a case is already uncomfortable to hold, and if the battery were directly integrated into the phone, I could get a little more battery in the added thickness. One option I wouldn't mind seeing more of is phones with removable back covers, to let you replace the battery with a larger one, while giving people who want a thin device the option to stick with the stock cover.