Native platforms have been, and still are (thanks to Apple's success), the major roadblock to innovation.
And your complaint isn't even accurate aside from that - DARPA funded SRI for military purposes, SRI finished their military contract and reformed to a commercial company and Apple bought them (I.e. innovation happenings) and in an interview with the company founder he said they had to struggle to get it to run on the 3GS and implied it would be much better on better hardware. Speculation is that it does voice recognition on the client (I.e. There are plausible technical reasons why not every smartphone could run the current implementation).
Having to differentiate their native platform drives innovation, its having to support the lowest common denominator and backwards compatibility which stifles it, and patents which roadblock it.
Your comparison isn't valid.
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 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.