Defining "usable life" as continuing to receive Operating System/ Security updatesI'm sitting here in my living room with an iPhone 3GS, an iPhone 4, an iPhone 4s, an iPhone 5, an iPhone 6s and an iPhone 6s+. All of which work. PS, my daughter uses an iPhone 5 on an active line as her primary phone.
I think you may need to revise your max useable life criteria for the iPhone.
Also, right next to my Android phone is my 2009 HTC Touch Pro. Granted, it's WinMo and not Android, but it is 11 years old and still functioning.