Well according to a few things that I have found, the average that people currently upgrade their phones is about every three years
https://www.techtimes.com/articles/...wait-3-years-to-upgrade-their-smartphones.htm. Specifically in iPhone users only about 20% of all iPhone users as of 2018 upgrade within a year
https://9to5mac.com/2018/03/12/new-...ll-upgrade-to-a-new-iphone-iphones-this-fall/
Apple also only sells between 180 and 190 million iPhones globally a year
https://marketrealist.com/2019/08/iphone-loses-global-smartphone-market-share/. Of those only about 60-70 percent of the sales are of the newest model
https://thenextweb.com/plugged/2020/01/21/apple-iphone-11-sales-q4-2019-prediction-analysis-se-2/. This means that when an iPhone model is the brand new model, Apple will sell about 108 - 133 million units of that model. In order for the number 50 million to be correct then that would mean that between 40 and 50 percent of all brand new iPhone model sales in a year would be to people that bought the brand new model the year before. However, only about 2 percent of iPhone users upgrade their phones every year
https://www.statista.com/chart/3634/smartphone-upgrades/. This means that the amount of people going out and buying the new model every single year would be 2% of the 108-133 million new models that are sold globally on a yearly basis.
At most 2.5 million people upgrade their iPhones every year of the roughly 900 million total global iPhone users. 0.27777% of iPhone users upgrade on a yearly basis, that is a niche group.