IMO not a good excuse given how expensive iPhones are, not bringing software features after just year is poor from a company which claims to offer the best software supportSince the Photonic Engine uses Machine Learning and since its ML model would be dependant upon the camera/sensor, I doubt they would spend the money/developer effort to backport that to any other device that matched only on CPU requirements (e.g., an iPad that had the same CPU).
I know it can be frustrating/disappointing, but if it makes you feel any better, this happens with every generation (including features that seem much more trivial to backport.)
Since the Photonic Engine uses Machine Learning and since its ML model would be dependant upon the camera/sensor