If you owned an iPhone before you will have a charger and a cable usable for charging your new iPhone. This will remain true until Apple makes the switch to USB-C everywhere according to EU regulations Then you may need a new cable (USB-A to USB-C). I can't see what is forcing you to buy an USB-C charger if you have one with USB-A. Until that thing goes down and stops working.

If you use the trade-in option with Apple you are required to return your device only, you can keep any accessory you got along with it. In terms of efficiency it is better to continue to use a charger than to return it for recycling. This does not get wrong by the fact that Apple may profit from the savings of not producing a new charger. Where is your logic? There is none in your argument. You relate a maybe-fact of Apple making more profit by doing less to the environmental impact and conclude that there is no improvement of the latter. Without any reason.

May I say that I find the option to return a not-needed charger packaged with the phone for free idiotic? Damage ist done, thing is produced, transport back and forth is needed and the fate of that charger is than what? Being included with the next box, just to be transported back and forth and so on... Not producing unneeded stuff in the first place is most efficient.