I think this is an insightful comment - but I think you underestimate how many people "want" Siri. It's gotten great media exposure - like the recent Big Bang Theory TV episode where one of the characters (Raj) falls in love with Siri, and Apple's advertising for Siri has been excellent. I think there are a bunch of people who "want that phone with that Siri person on it". You may be right about people never using it again... but a lot of people want it just because it seems futuristic and they've seen lots of media exposure. I know at least three people who bought a 4S because they wanted Siri, and only Siri. Android users will roll their eyes that people are going out to buy a phone with a feature that Android has had for a while... but you have to admit that Apple does a great job advertising, and I've never seen an ad about the voice recognition features of Android.
But you are probably right that lots of 3GS owners were eligible to upgrade (~18 months) so that's one of the biggest drivers of sales... and it's something that didn't occur to me.