The new Apple Watch Series 9 has a brand new processor with better on-device ML (machine learning) capabilities, unlike the previous 3 generations that were minor iterations of the same Series 6 processor.
As for the camera improvements, are you talking about the "regular" iPhone 15, or the Pro model? The Pro model has the new A17 processor, whereas the 14 Pro and the regular 15 have the A16 Bionic. Different processors, different capabilities.
So it's not some arbitrary software-lock. There are actual hardware differences that matter.