Why make it look like a resemblance. It's better to not make it edge to edge because the way it will shatter close to corners and how much less there is to grab the front.
But then their names would be messed up, since the didn't release the 6s yet, and when that comes what will the 6c be, 6cc??? They should just release the "c" every other year, when people with older "c"'s can upgrade
That's how it already works, first they release a new iPhone upgraded from a previous model, then a year later the c and s models come out, for years it has been s but higher price for a bigger phone but cheaper would be about iPhone 5s' price. Therefore this may not be necessary because 5c will be cheaper than 6c (if that were to happen).