The problem occurs when:
1. Holding the phone in my right, I try to press volume (I naturally do this with my index finger, especially for the up volume button), at the same time I often unintentionally press the power with my thumb.
2. Holding the phone in the left hand, when I try to press the volume buttons (with my thumb), my index finger unintentionally presses the power.
Whenever I should reach the up volume button the way you say is natural (thumb of the left hand or the middle finger of the right hand [since you say your index finger on your right hand is behind the phone]) then, in such hand position, I can also perfectly well reach the top of the phone with my index finger (of the hand that is holding the phone).
So, I insist putting the power button on the side is a non sense and it's the thing that drove me crazy on an android phone I had for a while as a second phone. Back then I thought: what idiot can put the power button here. One iPhone generation later, Apple joined the idiots ;-).