Siri's servers were falling over just handling the people who had 4S devices in their hands, imagine unleashing all iPhone 4 and iPad 2 owners on their servers as well at the same time...
So far Apple has not come out and said that Siri will not be available on older devices (such as the iPhone 4 and iPad 2). I honestly believe that Siri is very much still considered a beta product by Apple and they are attempting to figure out how to make it scale on the backend.
I bought the iPhone 4S, upgraded from an iPhone 3G and I would be more than happy for more users to get Siri because it is absolutely fantastic. I would love to get it on my iPad 2 as well for example. If more people have Siri and use it, it will most likely improve faster, we will hopefully get API's to program against, and it will get better and better at understanding or processing commands.
I don't see how it would be a blow for iPhone 4S owners ... they got early access. The iPhone 4S pretty much stands on its own, the dual-core processor definitely helps, and the increased speed in graphics makes games and apps look absolutely fantastic.
No more than it's a blow to me when the movie I paid $10 in theaters for winds up in Redbox for $1. If your enjoyment of a device is predicated on its features not being available to others, you're in for a world of disappointment.
How did you read that on my words? That's not even a good comparison. A better comparison was when the original iPhone got its price reduced by $200. It was a blow and Apple acted accordingly. If Siri were to be available tomorrow for iOS 5 devices when most people didn't even get their iPhone 4S I would expect it to be a blow to a good part of the early buyers.
So far Apple has not come out and said that Siri will not be available on older devices (such as the iPhone 4 and iPad 2). I honestly believe that Siri is very much still considered a beta product by Apple and they are attempting to figure out how to make it scale on the backend.