It's not just an issue of poverty, as if only poor countries have to contend with old phones. In rich countries, there's still an issue of valuation. I have money, but I don't value phones that highly. Just like I could buy a $100 ice cream if I wanted to, but I don't value ice cream that highly.
The effect may be indirect - someone in a poor country with a 6 year old iDevice finds it becomes unusable, so they want to buy a 3 year old iDevice... And they're buying that from someone who has a 3 year old iDevice who sees that for not a lot of extra money they can buy the newest one.