Especially on a phone where the life cycle is so short.
Not as much for iPhones.
I mean, think about it- the 2011 iPhone 4s still runs a modern iOS! I have gone through two subsidized upgrades since that thing released, and I know many people who still have a 4s they use.
What benefit does Apple get from extremely long life of devices? At one point it helped build an install base, but now with a more saturated market that means less sales than they otherwise might have. You can't tell me that Apple isn't looking over at Android, where if a phone is supported two years out it a miracle, and is licking their chops. With Next plans giving more of an ability to upgrade Apple needs to give people (who otherwise might be happy with the large screen iPhone for 4+ years) a reason to upgrade.
PLUS there is the whole fact that quite honestly the big phones were back peddling. Steve told us anything bigger than any average hand can use (aka 4 inches) was out, and yet here we are and Apple blew past that old "optimized experience" without so much as an excuse. In fact, they did the opposite and had commercials that just rubbed in the bigness in a very not-Steve fashion.
For all the fanboys of small phones the fact that the 5S will probably have the EXACT same lifespan as the new iPhones (same RAM, same Power-per-pixel, both 64 bit) is a nice consolation prize for no longer having a flagship option. In fact, I think it is easy to say right now that the 5S might be another four year device, but the 6/6+ won't be (or at least the usable life won't be).