Being green and environmentally conscious means using your phone, your car, and everything, for as long as possible, and buying as few things as possible, not replacing them with or buying a lot of unnecessary new things that have an "ECO" sticker on them.
If Apple really wants to get green credentials, they should keep releasing security updates for old phones, independently of whether the OS is old and cannot be updated.
An iPhone 5s on its last available OS with security updates is a perfectly functional and capable device. Imagine updating your iPhone yearly for 10 years instead of keeping your 5s, what amount of materials (mining, processing), manufacturing (energy), and this also means capital goods and the creation of those, plus logistics have you used unnecessarily?
Enough resources to run a well-optimized home for several years. And that's just your phone.