The older iPhones still hold resale value pretty well too, which means most iPhones have a second life as a "cheap" option. Combined with the software updates you mentioned, that's pretty good for the environment. The best case scenario is most iPhones get to be used by multiple people and new iPhones contain a lot of recycled material. That's not too far from reality.

The problem with tech is nothing is ever "good enough." Sure, you could make a phone that is durable for 10 years... but someone will invent new battery chemistry, CPUs will get considerably more powerful/efficient, and Sony will come along with even better camera sensors.

So it's really not possible for that high-end "old" phone to keep up after a few years. That's true for most consumer tech, even if the pace is slowing down somewhat. An expensive LED TV from 2013 will not compare favorably with a good value mid-range one today.

We can either stop innovating, which is also bad for the environment (technology improvements allow for vast efficiency and energy-use improvements in a huge range of products), or we can make sure things have a lot of re-use value. That can be done through the used market with good repairability and software support, and by making sure phones can be recycled and also use a lot of recycled materials.