Apple should upgrade their products whenever they think they have improvements that are ready technically. I still have an unlocked 4S and love it with no need to upgrade to a 5 or a 5S unless some compelling new feature arises. Maybe Jony Ive will introduce new UI software that the 4S won't have enough horsepower to run, but I doubt that. It will be an iOS upgrade that just runs fine on the 4S. If not, I'll upgrade the phone.
I feel less of me - I usually update annually but this 8 month BS hurts re sale values.