You will still have phones that come with “standard USB C” cables where some support different power delivery, data speeds (or not support data transfer at all), very few will support video over USB C (which is standardized), etc.
So still when the Android user with the cheap power only USB C cord moves over to the hypothetical iPhone 15 with USB C, they will still have to throw away their USB C cable that potentially doesn’t support high speed data or video over USB C. Both supported by todays iPads with USB C.
If the same mandate had gone through when it was first proposed, we would have been stuck with micro USB.