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It's not going to be a surprise at all, I'm thinking it's expected and a big reason of making the 5C in the first place. They knew the iPad Mini was going to eat sales from the full size iPad. I'm guessing they have run the numbers and expect the greater sales numbers in whole will result in greater ultimate profit even when you take out lost 5S sales.

Customer satisfaction is also going to be higher as a 5C with rumored specs is going be a lot better experience than the 4S and a massive upgrade of the 4 that were selling around the same price point.
 
The real reason for the 5C is to make sure the entire lineup is 16:9 and has Lightning connectivity.

Current lineup
iPhone 4 - $0
iPhone 4S - $99
iPhone 5 - $199

New lineup
iPhone 5C - $0
iPhone 5 - $99
iPhone 5S - $199

The fact that they can also sell the 5C in developing markets is a plus.
 
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