Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3)
That might be true but he's correct to his point that it's irrelevant. This is a product thing, the technical part doesn't matter like you think it does. As far as a product goes, Apple doesn't have to give anything away for an old product when it wasn't ever represented as having it, regardless of the technical capability of running it.
Software development costs money, just like hardware production does. Apple has no need to give away software to enable your iPhone 4 to use Siri just as, for example, Subaru wouldn't need to give you snow tires for your older model car just because the new one comes with them, but is otherwise the same car.
Just because the software could work does NOT mean it should be free. In the case of the phone or the car, you're able to go add the functionality yourself if you have the ability, or you can go buy the new product. You shouldn't be expecting handouts. That's not reasonable. The product you bought does at least everything it was sold with originally.