I think "most people" have been shifting towards longer than 24-month replacement cycles. Leasing with a fixed end is probably predicted in their financial models to do a better job of getting more people onto a strict 24 month cycle. Nobody will want to actually buy out their lease on a 2-year-old phone, so they'll turn it in and lease another one, so Apple can throw it into the shredder[1] and show an additional sale on their books.

[1] https://www.ifixit.com/News/94386/the-truth-about-apples-fre...