If iOS 7 was release tomorrow and the 3Gs did not support it and Apple stopped signing the latest version of iOS for the 3Gs, 6.1.3, it would render the devices mostly inoperable because end users would not be able to restore their devices. Just like if Apple was to stop signing iOS 5.1.1 for the iPad 1, which is also the latest version of iOS for the iPad.
I have no idea what you are talking about. If they were to release iOS 7 Tomorrow and the 3Gs didn't support it, you would simply restore your device to 6.1.3 and not iOS 7, the same way that your iPad 1 only works on 5.1.1 and not iOS 6
I'm sure part of the reason Apple does this is to get people to purchase a new device. They don't make any money off of their OS. iOS 6 is a free download. They are in the business of selling hardware.