My brother came out of college with a math degree focused on statistics. He got a job with a cellphone conglomerate. We all think there are a bunch of cell phone makers but in fact there are (were at the time) about 3 who manufactured brands for all the distributors.
My brother develop algorithms on how long people could keep their phone before a "free" upgrade. The manufacturing cost of a $1000 iPhone is like $100. The idea is to get the consumer on the hook by contracts, financing phones. The question is how to keep them. The "reason" an iPhone costs $1,000 retail is because it makes it really, really, really hard to pay it off and switch carriers.
3-4 years? Not bad. I kept my last one about 10 years and the current one is about 4 years and going strong. I cracked the screen and while I usually view this kind of tech disposable it was only $75 so I fixed it.