I'm running a 64gb iPhone 6S, about 3 years old. It's starting to slow down just a bit with age and software updates but I'm wanting to hold onto the phone until it's either unbearably slow or inoperable. I like iOS a lot but it's a big deal to me that Apple removed the headphone port so I know an iPhone upgrade will be painful.
I'd say just switch to Android but I really don't like the OS. Guess I'll have to revisit the idea when the time comes.