Design Flaw
While I wouldn't ever put my phone in my back pocket, and I'm not concerned with my 6 bending in the leather cover as that adds rigidity (much stiffer than non-Apple leather cases/covers), this is still a design flaw.
Poorly thought out changes from iPhone 5 to 6:
1. Moved On/Off button from top to side, opposite volume buttons. Not only did this line up two week points, a no-no in structural rigidity, it makes it difficult to turn off/on without accidentally adjust volume and vice versa.
2. Volume buttons turned into ovoid bars instead of round buttons. These ovoid bars are twice as long as the old round buttons. They end up creating a very week side of the phone. Long ovoid voids in metal are very week. Round holes in metal, as long as they aren't too large, do very little to impact the strength.
3. Volume buttons now directly opposite SIM slot. So now you have 5 long ovoid voids in the structure (mute, volume x2, on/off, sim) all opposite each other. Very bad design.
4. Made phone too thin. Every millimeter counts in rigidity. An extra millimeter or two would have increased the strength of the phone a very large amount.
Why Apple did this?
They claim that the on/off was too hard to reach 1 handed if left on top. Not sure about the +, but on my 6 I can reach the top of my phone with my first finger rather than thumb to sleep it. No need to reach it to turn it on as you don't press that button to do that.
Volume buttons changed for purely aesthetic reasons. Was it worth it? Hardly.
Thinness is because Ive is obsessed with it even if no customers are asking for it. We asked for better batteries. In the 6, 1 mm would have allowed for 1 mm more of actual battery guts, and at least a 25% capacity increase. My 6 doesn't have enough battery. It's about what the 5 was, and considering it's bigger, it makes no sense.
How Apple can fix it in this iteration:
Resize the buttons in question to be much smaller with round holes. Round holes are much stronger. They can still line up with the case buttons, though the buttons now won't be backed by a full sized button underneath.
What they can do with the next version:
Move the on/off button, decrease the size of the volume buttons.
Make the phone 1mm thicker, and advertise how much better the battery is.