If we disregard the whole 'planned obsolescence' theory, it all comes down to one integral aspect of iOS that should be of the highest priority: optimization.
Clearly iOS is only properly optimized for the iPhone 13 models and this could be fixed later on, as Apple software seems to have been coming out in a half-baked and unoptimized state nowadays and then fixed later with an update (e.g. ProMotion on 13 and MBP, webcam quality on Studio Display).
Don't misunderstand me, I am NOT condoning this kind of ****** software quality (especially coming from a vertically integrated company like Apple), but I am still positive that a fix is coming at some point.