One way to interpret this move is to conclude Apple is taking the easy road and is merely recycling an existing chassis design to produce a lower-cost iPhone without hurting margins. Another possible interpretation is that they are reviving the iconic iPhone 5/5s design in an attempt to cater to a group of users who not only want a cheaper iPhone but would also prefer the smaller form factor. After all, a third of Apple users are still using older 4-inch smartphones.
