iPhone 3G was supported until 4.2.1. Why would the 3GS not get 5.x following the same path ?
What part of the iPhone 3G being strained by iOS 4 do you choose not to accept? I wasn't sure if you were asking for the 3GS to receive iOS 5 in its entirety or at least a lighter version, in which case the latter would be reasonable, but it sounds like you're hoping for the 3GS to fully support iOS 5 like the 3G did with iOS 4 (for a while at least).
If that's the case, then why are you so adamant about getting iOS 5 if it might not function as well on your phone? Or are you insinuating that iOS 5 should be dumbed down for the iPhone 4 and 4s/5 just so it can be "fully supported" for the 3GS?
Either way, it sounds like you're just complaining for the sake of complaining. If Apple releases iOS 5 in its entirety for the 3GS and it slows it down like iOS 4 did with the 3G, then you'd probably complain about your 3GS not functioning as efficiently. If Apple dumbs down iOS 5 for the iPhone 4 and 4s/5 so it is "fully supported" by 3GS *without* hampering it, then that's just selfish to restrict the next two iterations just so you can say that your one model is "up to date" when your model already got its upgrade from 3.0 to 4.0.