With most consumer products, many times, there isn't much of a change to jump from Product 2011 to Product 2012. iPhone isn't any different. Maybe there's enough of a jump to go from Product 2011 to Product 2013.
And that's exactly the point!
The iPhone 4S doesn't have to be a completely different device in comparison to the iPhone 4 - it simply has to be much better than the iPhone 3GS.
The same with the iPhone 4 and 5 - the 5 doesn't have to be completely different than the iPhone 4S - it has to be much better than the iPhone 4 - that it is also much lighter, faster and has tons of other features the iPhone 4S doesn't have is just an added bonus.
Most of the iPhones will be sold with a contract - it is the norm in the US and it is also the most prevalent in Europe.
Contracts which run for 24 months... 2 years... the timeframe between iPhone 2G and 3GS, iPhone 3G and iPhone 4, iPhone 3GS and iPhone 4S, iPhone 4 and iPhone 5.
And all of these leaps have had lots of improvements, the devices you would get when you replaced your old iPhone were always much better than the old device.
It's funny - a few years ago, the Anti Apple-crowd was complaining that the "iSheep" would buy a new device every year to feel superior and to show off.
Now the same Anti Apple-crowd is complaining that Apple isn't innovating enough to make people upgrade every year (which isn't possible in many contracts).
So, if people upgrade every year, they are iSheep, if people upgrade only every two years, Apple is doomed...
Maybe Apple should shut down the company and give the money to the Anti Apple crowd^^