How would you feel if the next iPhone had the sleep/wake button on the side on the Phone? I can not even begin to tell you how mad I would be. It's just so instinct to press it on the top. I really don't want the next iPhone to have it on the side.
If the next iPhone is bigger, having the button on top would be very annoying. You don't know how annoying until you've used a 4.7+" phone with the button on top. On a 5.5" phone, having the button on the side is simply necessary.
Having the button on the side is completely fine, you get used to it in a day and then you don't accidentally press the volume buttons any longer (given that they are on the opposite side). Trust me - I thought the side button would be annoying but after one day with my Note 2 I was used to it and didn't question it any longer.