I would have kept using the old phone in every case, but: they each became too slow with os updates.
In the case of the 6 I think it was exacerbated by battery issued. So probably you've taken better care of your battery? (Or stayed off ios 11)
But, if you look at that record, it is:
1. Not an upgrade every to every model 2. Still a fair number of sales for apple
I would have kept using the old phone in every case, but: they each became too slow with os updates.
In the case of the 6 I think it was exacerbated by battery issued. So probably you've taken better care of your battery? (Or stayed off ios 11)
But, if you look at that record, it is:
I skipped the 3g, 4s, 5, 5s, 6s, 7