Phones could easily last a decade.
The only problem with it is the battery. It could use a replacement, but since I don't use it as the main device, I don't much care. Otherwise, it works perfectly fine.
If you primarily care about the environment when getting a phone, the iPhone is the clear choice.
Except most people are not doing that.. I've seen quite a few iPhone 6, & 6s around and those devices are 7, 6 years old respectively. Its not too much different to the car industry, you will always have a class of people who like to chop and change, they may be more the enthusiast market, but many will just use it until it dies or becomes an issue and repairing it isn't worth it.
In my circles buying a new iPhone every 2 years is fast becoming a thing of the past
If the phone doesn't break during the two year warranty period, I've found that it's unlikely to break later on. (you have to treat it well, of course...)
One important reason why you want that original part and not a salvaged or non-OEM one for the battery is safety, this is a device that you carry on your person and the batteries carry a lot of punch. But the car industry has shown the way here, I can buy OEM brake discs and pads for my vehicle, I can also buy them from brand name non-OEM suppliers aka the 'aftermarket'. A similar solution would work well for phones, as would some minor design changes to facilitate repair rather than the opposite: design changes that make it harder to repair a device.
I see your point, but I don't like the idea of forcing companies to sell you things they don't want to.