It's not like your device will stop working the day iOS 7 is released. Your phone won't have any features taken away and you can keep it using until the hardware fails. (I'm still using my Nokia N900).
A lot of iOS 6 features rely on Apple infrastructure, e.g. iCloud, which will probably become even less reliable on iOS 6 as the months roll by. So no, the device becomes less useful. Try using iTunes on Lion if you don't believe me.
