That's assuming that the buyer is an existing iPhone owner, who is keeping the old cable. Apple seem to pride itself for having first time iPhone users buying their iPhones.
Even in this forum, I'm seeing people selling or giving their old iPhone away, highly likely with the old cable.
Anyway, this is getting a bit winded. We can agree to disagree. If this rumor is true, I don't like it, and it will only create more waste as Apple themselves have to package and ship their USB-C chargers separately instead of inside the iPhone box. I don't think this is a good move. Hopefully it's an incorrect rumor.