Except, "Siri" wasn't truly hardware dependent. Developers in the jailbreaking community easily ported "Siri" to unsupported devices.
Apple has generally tended to announce new features as hardware dependent to sell more devices. That's great, it's business, but a slight slap in the face for us tech savvy enough to know better.
I'd rather new devices have significant h/w updates aside from slight processor and camera tweaks. NFC would be a big one; whether you agree or not, it's been used and popular for years in Europe and especially Japan. N. America has some catching up to do.
Technically, Siri was hardware dependent. Yes, it could be ported to any device with a microphone, but in those cases it's performance would be adversely affected due to the lack of a designated voice processor that wasn't included in any IOS based devices until the 4S