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1) Cellphones don't just last 2 years; lots of people have older models. Sure, a bunch of people get rid of theirs after 2 years, but then they go to the secondhand market,Increasingly less so. On one side, you have cheap subscription plans that bundle usable (not too decent) phones, on the other hand, phone life is shortened by a combination of planned obsolescence, software bloat, and lack of security updates. Short sales cycles is what manufacturers plan for and actively work towards.
> Frequently, older phones eventually find their way to developing nations where people can't afford the latest iPhone.
I might be wrong, but I'm not convinced. Shipping costs money, phones are cheap, and both battery and flash memory have pretty short lifetimes now. A good chunk of phones are barely operational after 2-3 years.
RE benefits of smartphones, it's all true, but you can't run economic growth on it forever. Smartphone transformation happened. It's mostly done. Itself it won't fuel further growth. Production of hardware and software might, but that is not free in terms of energy/resources.