"Wahh... it's forced obsolescence!"
Boobs. The iPhone 4 is a fair amount faster than the 3GS and also features more RAM (an important consideration). The 3GS is starting to show some of its age running the current version of iOS. If iOS 5 makes significant improvements which require the greater processing power of the iPhone 4 (and any future generations) it would just make the 3GS even slower. I'm sure some of the same people complaining would be among the crowd growling at the speed decrease if Apple did offer support.
Apple has a pretty decent history of backward compatibility with its devices and in the cases where it is fairly borderline the fact that they do move on rather than retain backward support with everything is an important factor of what allows them to innovate so quickly.
If the features of iOS 5 are must-haves for you, upgrade your device. Or continue enjoying a device which will continue functioning as reliably as it has in the pastand with all the features youve been enjoying since you purchased it.
Great post. The other thing to remember is that the iPhone 4 is required to run HD apps. Surely this tells us that the iPhone 4 does not contain the same hardware as the preceding models, and further highlights that Apple are looking to standardise things. The iPhone 3GS being still sold does not reflect that Apple are going to upgrade the OS for it indefinitely, merely with it's pricing it shows that it aims to price it for those looking for a bargain iPhone and nothing more.