As I posted a couple months ago, here is the "workaround" for this problem that works fine for me. It enables me to at most use the power button only once per day (to avoid wearing it out further). I find this method is quite convenient....
1. On you iPhone, go to Settings->General->Accessibility->Assistive Touch. Turn on Assistive Touch.
2. When the power is turned off on your phone, and if you still have some function avaialble with your power switch, perhas only on the left side, use the switch to turn power on. If that doesn't work, then plug in your charger and that will definitely turn on power to your phone immediately. As soon as its on, if you don't yet need a charge, then unplug the charger. (In my case, I turn the phone's ower completely off every night and turn it back on in the morning. Only once in the past 2 months have I had to resort to using the charger to force a start-up.)
3. You will notice a "snowball" icon on your screen, which you can move anywhere that is convenient. When you want to put your iPhone to sleep, just tap the snowball. A menu will appear - tap n the "Device" option, and then tap on the "Lock Screen" option and the screen will go blank.
4. To wake up your sleeping iPhone, just press the Home button at the bottom center of the phone. This will wake up the phone just like the power button.
5. To turn off power to the phone, tap on the snowball icon, select "Device" and then HOLD DOWN the "Lock Screen" option icon until the sliding red button appears saying "slide to power off." Slide the red button to the right, and the phone will power off.
6. To power-up again, try the power button. It might work, if pressed a couple of times on the left end of it. If it doesn't work, then briefly plug into your charger, and the phone will start right up.
Though this method has been criticized here, it works fine for me, is convenient, and will keep me going until October when AT&T will let me upgrade to another phone, maybe an iPhone 6 - because in spite of this problem, I really like my iPhone.
Why don't the "geniuses" at the Apple Store know this work around and tell it to people to ease their frustration? Because 1) the "geniuses" foolishly don't read this blog, and 2) they're not geniuses.