If you bought a previous generation iPhone AFTER the next generation has been released for over 6 months, and you expect the kind of support you would have received when it was new...of course you're going to be disappointed. The 3GS will NOT get any substantial feature updates from here on out, it's prime is long post, the iPhone 4 will see this same downward trend now too and that will follow straight through the release of the iP5.
Your logic in this seems a bit flawed just based on what you asked the Apple store employee...you asked them if the 3GS was still being made, how does that, in ANY way, reflect on the support and adding of features to the phone in the future? You bought a phone that is approaching 2 years old, and as we've discussed in other threads about macbooks and, in this case iphones...it's ancient in technology years. Is it still a great phone? Absolutely! That's why i sold my iPhone 4 and bought my 3GS and i'm totally content with it...but i'm also well aware that this phone will not be supported with any legitimate feature additions, and will simply be supported in a very basic sense from here on out...ESPECIALLY after the release of the iPhone 5.