I don't see how one end already being USB-C makes a difference. If either end/port is different, you need a new cable.
Because MacBooks and (newer) iPads are such a proportionally small market compared to iPhones, I can't agree. The best way to avoid environmental impact is: reduce, reuse, recycle -- in that order. A switch to USB-C does not allow for reducing or reusing, and would in fact do the opposite: creating a surge in demand for new cables and chargers to perform the same function that Lightning hardware that people already possess today already does.
I have two iPhones, iPad, Apple Pencil, AirPods, and a MBP; and only the MacBook Pro is USB-C -- the rest all have Lightning connectors. As a result, I've got numerous Lightning cables and USB-A chargers connected to them scattered around the house and in my travel bag, but only two USB-C/TB3 cables and one USB-C charger (the one that came with the MBP).
As a result, switching from Lightning to USB-C in the iPhone, iPad, and AirPods would just leave me with a bunch of Lightning cables and USB-A chargers that are no longer usable to me and need to be replaced with functionally-identical USB-C cables and chargers. That's unnecessary cost for me and the creation of waste as those Lightning cables are still functional, but rendered artificially useless by Apple's choice to adopt a new port.
The point being that I'm certain many others are in the same situation because most Apple devices to date by quantities sold have a Lightning port, not USB-C. The result would be that millions of people would end up with a large amount of Lightning waste that they end up deciding to throw away because they had to go out and buy redundant USB-C replacements, and those Lightning cables aren't and won't be usable with any new devices going forward.