Leaving it out in the cold? Such drama.
IMO, it was a mistake to provide iOS4.0 for the iPhone 3G to begin with. Multi-tasking with 3rd party apps is not supported along with a few other features that are supported with the 3GS and iPhone 4 with iOS4. The iPhone 3G has the same processor and RAM as the original iPhone. It doesn't have enough processor and RAM punch for iOS4, which has caused problems for some 3G owners with many ranting and whining about it here. Two years is a long time in the computer industry and especially in the cell phone industry - at least after Apple entered the market. Prior to that we pretty much saw the same old thing for the most part but there were new cell phone models released on a yearly basis with no updates provided for those phones.
If the 3G is not supported with 4.3 now, it won't be supported later. And the features you are probably interested in that are included with 4.3 for the iPhone 4 would not be compatible with the 3G anyway.
The iPhone 3G was released in the summer of 2008, which included iOS 2.0. The iPhone 3G was included with all iOS 2.0 updates, iOS 3.0 and all updates, and iOS 4.0 with limited features compared to 4.0 with the 3GS and iPhone 4 along with all 4.0 updates until 4.3.
Being left out in the cold? Not hardly, but hardware limitations will prevent a device from being included with a firmware update eventually, and the same with computers with major OS updates or upgrades. I venture to say the overwhelming majority of people get a new cell phone every two years or so.