Apple needs to stop selling outdated technology. They're selling the iPhone 4s and the iPad 2 which won't even be compatible with iOS 8 next year. Hell, the iPad 2 is barely compatible with iOS 7. It runs like crap.
I guess there are several ways of cutting costs to be able to release a "low end" phone. One is to throw together a phone that didnt cost too much to design using cheap materials and parts. Another is to use existing designs and production lines and older gen tech. Once you have a mature production line running smoothly, it doesnt cost as much to keep it pumping out products.
I guess Apple feel that the latter fits more with their idea of customer experience of an iphone.
----------
More margin with the 5C compared to the 5, and more for the 5S because of the higher price.
It's just a replacement because of the margin.
I dont know whether there is a higher margin on the 5C compared to a last gen phone - i dont think apple release that information. But even if its true, and I dont doubt that they try to maximise margins (the would have to answer to shareholders if they did otherwise), i think its simplistic to say thats the only reason.
I know there are many people who like the addition of a more "fun" iphone, and many people who prefer plastic over the easy to damage metal. I believe that there are also still supply chain constraints on the 5/5S aluminum shell. It wouldnt make sense to use up production volume on their mid range phone, while taking away from their flagship.
The final point, that most people seem to ignore, is that the 5C is a starting point for their mid range phone. Apple have never come out and said that every year they will repackage last year's phone in a plastic shell and sell it as mid range (in fact, they didnt do it this year as there are some other changes). Most likely the "C" line will develop independently of the flagship line based on what the "C" line customer base look for - it will likely get different functionality and may run on a different update cycle to the flagship.