It is possible that Siri will eventually be available to the iPhone4 and iPad. No one really knows. And no one can say for sure what the reasoning behind it is. You can speculate that it's because the iPhone 4S needed more features, but honestly, I've used my husband's 4 and my 4S is noticeably faster and smoother. So saying that Siri was needed to help the 4S be a success isn't really true. I think there is probably some truth to what rwilliams and cyks said. Siri has been up and down since launch day because there's so many people trying to use it and it's overwhelming the system. So it does make sense that Apple could have limited the people it was available to in order to help cut down on those problems.
To the OP, I understand that you're upset that they took the app away. At the same time, it was a free app, and I guess I always assume that what is free can easily be taken away. It's frustrating and it sucks, but I don't think it means that Apple is headed in the wrong direction overall. People keep posting about things that are happening "post-Jobs." In reality, all of the things that are happening were known about by Steve. Most things that come up in the next few years will still be things that were known about by Steve. Apple has not drastically changed in the past few weeks, so please don't blame it on Steve's death and the new management.
I think Apple did do enough to notify people that the Siri app was going away. If you had questions about what the "going home" message meant, a Google search would have solved it. No, that wasn't the main point of your original post, and I understand that, but you did complain about them not notifying you. They did...they tried to do it in a cute way, rather than being extremely overt about it, but they still did notify people. Wait and see what happens. After things settle down, Siri may appear on the iPhone 4. If not, there is other software than can be used in its place. Apps disappear all the time, and while it's frustrating, I'm not sure it's worth being so upset about it.
Hopefully this post doesn't cause any more arguing. Just putting my thoughts out there.

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that doesn't explain losing the existing app which worked fine on the existing servers and iPhone 4 hardware.
That would mean splitting up the development and support, though. People would have to work on the App and on the new 4S version of Siri, instead of being able to focus on just one. That does complicate things.