You're just assuming that, but iPad 3 is months away and we don't know how sales will hold up over these next few months. If apple holds out on things like Siri for iPad 2, that doesn't look good to consumers and is an incentive to not buy an iPad 2 now and instead wait.
Cripple old, not top-of-the-line products? Sure, makes perfect sense. Cripple one of your flagship products to try and make a product that's months away from shipping look better? Sorry, I don't buy that. Especially when months from now people will still like Siri but it won't have the wow factor to be the main selling point any more.