Initially I shared your sentiment, as my iPhone 15 will not support AI either. However, by now I have totally relaxed about this whole AI business, as it is still in its infancy and will take at least a year to be fine-tuned, by which point we will probably be reaching iPhone 17 release. ChatGPT app should work as before and Siri should also improve somewhat. We shall see by how much once iOS 18 launches for all in the autumn. Basically, I am not upgrading my iPhone 15 until at least iPhone 17.
As others have mentioned, the problem with pre-15 Pro iPhones is their sub-8GB RAM limitation, which is the absolute minimum to run any form of AI model on device. So no, it is not just about the money.