Anyone who has this issue whose phone is under warranty should definitely just get it replaced! I'm glad you had a good experience at the store. The problem is that most of us are out of warranty (unless one purchased AppleCare) as the phone first came out in the USA ~20 months ago. Please note that you don't have to go to an Apple Store to get your phone replaced under warranty. Apple has world class telephone technical support and is able to ship replacements to most any country in the world. They will walk you through the same things over the phone that the "Geniuses" try in the store. If they are unable to resolve the problem with you, they are more than happy to ship a replacement most anywhere in the world.
As far as this statement goes: "So I do recommend NOT to alter the device to avoid losing the warranty! And even if your warranty is expired still better not to stick anything into the device as you may break inner parts where then it'll cost you more money!" Those of us out of warrantly have absolutely nothing to lose. If we break it, we'll be paying Apple $200 for a replacement phone. If we take the phone with a manufunctioning home button to Apple, they'll tell us all they can do is replace the phone for $200. We are not out anything by trying to fix it ourselves. In any case, I assure you, placing electrical tape on the inside of the rear panel or cleaning the button with appropriate contact cleaner, will not break the phone. Even taking riskier actions, like replacing the home button, can't result in us being in any worse of a position than when we started.
