You nailed it. They intro a commonsense feature and people start selling their old idevice to get the new idevice that mysteriously is the one idevice that can support the magical new software feature.
I think is also largely has to do with their overall philosophy of making the user experience the most hassle free, "it just works", etc etc type experience.
Could siri technically be installed and work on older generation devices? Yeah, I'm pretty sure jailbroken devices could.
Could Apple implement all of the transparency and 3d-gyro effects on the older devices? Yeah (well, the ones with the gyro
).
But I'm assuming these would have an effect on performance. Maybe not huge in most cases, maybe worst in others... But regardless, it would change that experience those device owners have come to expect - the device they updated would then run slower after an "upgrade". I think Apple sees leaving these features out in two lights:
1) Yeah, it does entice people to upgrade to the newer model. Most people I know, however, don't upgrade their iPhone every year - this forum is an exception from that for the most part obviously.
2) If it was either ios7 transparency and slower performance, or no ios7 transparency and better performance, I personally think Apple tends to go with the latter. The perk of the new model is that is has the new feature, and is either the same, or better performance. There's no trade-off there.