I think you nailed it.So what DID the Iphone 5C accomplish.
1) It kept the profit margin higher by reducing production costs of the "Mid-range" phone by dropping the expensive enclosure.
2) It moved all the "Mid-Range" phone production OFF of the already taxed assembly lines of the Iphone 5S, thus allowing for higher yields of the, even more profitable, Iphone 5S.
Of course, just a theory...
I also think they got the colors wrong. Not the overall choices but the fine tuning. My wife wanted to get the yellow but when she saw it in the store she said it entirely the wrong shade of yellow that most women are looking for. Same with the pink. They went too muted in the colors, but if you're in the market for pink or yellow you are going to want the real thing. Note the sales figures from a few weeks ago that placed yellow at the bottom of the list.