The vast majority of people around the world that rely on carrier subsidies (let alone the ones that actually pay directly) are on a 2 year or more upgrade cycle. Why would they bring out a completely new case design every year?
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For the vast majority of folks who dont fit into your pulled out of the air statistic and that aren't on a two year cycle.
Not all countries are 24 months as you claim, here for example the standard contract is 18 months, with 12 being an option to some.
Likewise the iPhone 4S/5 will be 16 months since it was last updated, 18 months since manufacturing for previous model began on mass. So people hoping for a new iPhone are not demanding a new iPhone every year, 16-18 months is more than enough time to be waiting for a decent refresh.
Look at Samsung and HTC they have already bumped up/refreshed their Galaxy S2 and Sensation models with bigger screens again and faster CPU's up to 1.5ghz dual core now, and this only 3-4 months after their flag ship phone models went on sale.
Whilst no-one is demanding Apple incrementally do anything as stupid or polarizing as that (I'd be pissed off if the €500-600 flagship model phone I just bought 12 weeks ago was already being gazumped) it is increasingly clear to everyone, analysts included that a mere refresh to the iPhone 4S internals is not going to be enough.
Folks are fickle, tech reporters likewise. Only last Monday on the Irish radio show Newstalk and it's Technology spot on George Hook, we had a female 'techie' (personally however I would question their credentials as they mostly pass misinformation as expertise on that show) who once swore by Apple, now advising everyone to move to Android because it was faster, had bigger screen etc...
As much as we all know there is more to a smartphone than specs, (apps etc), the reality is specs and bigger screens do count for a great deal, when the general smartphone buying people go into a store and pick up their new handset.
