I feel like I dont need a new model every year. Im 16 years old, using iPhone since 11 or 12, not sure now, had two models (6+ and now 8+). Ive been using 6+ since february 2015 till december 2017, upgraded for no reason, sometimes it lagged a bit, but it was still ok to use it and I upgraded for no reason (If I dont count rare lags). Now using 8+ since december 2017 and I didnt register a single lag. Its still powerful enough. I have no problem with that phone, everything is running fast and smooth and if Ill change the phone this year, it will be only because of battery (have only 83%), not because of its performance.
People in Europe and NA usually leas it because they get phone + data for lets say 20-30€ which is very nice price that most of people can afford to pay. (Middle class and higher for sure). People from other continents probably leas it because they simply cant afford to pay full price.