The way carrier subsidies work in my country (Singapore), there is very little benefit to not recontracting to a new phone every 2 years.
Late last year, I just upgraded from a 4s to a 5s. A 32gb 5s would have cost $368 (I ended up with a 64gb iPhone with Applecare+) and sold my 32gb 4s for about $300 (might have been able to fetch a better price, but I am not the haggling sort).
Based on this, my biannual upgrade is practically free of charge, I pay the same monthly phone bill regardless of whether my phone is purchased via a carrier or unlocked from the Apple Store, and the added features and improved performance more than outweigh whatever small extra I shell out every 2 years.
Even if my 5s is still working great 2 years from now, I would still upgrade to the 6s because...why not?