The whole idea behind the "c" model is to move away from the perception the original model had - the 5, and soon the 6 - to make room for the "s" model, which shares a similar look to the previous, non-s phone.
To do so, they first wrap it in colourful plastic, which gives it a whole new look and a different type of appeal.
They also iron out any of the major kinks the original phone first had on release. So if the battery was notoriously bad, they'll take measures to fix that. These are tweaks more so than upgrades - the kinds of things that would have been in place from day one of the original if they'd just had more time to test and develop it before release. (Another important reason why the "s" model never becomes a "c" model - it's had an extra year of development compared to the original in the same industrial design configuration, so they have all the time they needed to iron out kinks, which is why the "s" models always tend to last longer and have fewer problems than their original, non-s counterparts).
They also reduce the storage configurations to two, lower end choices (so you won't see a 128GB 6c, for example).
And finally they lower the price one peg, into the mid-tier range.
The 6s takes top tier pricing with three storage size ranges and the 5s goes down to the lowest pricing with only one storage size (I'm guessing 16GB this time but it could well be 8GB).
You can think of the "c" as a "downgrade" to plastic if you like, but it's really all about differentiation from the "s" model.
You make some compelling arguments that are reasonable and well thought out. However, what is your explanation for the plastic shell with the 5s camera cut-out in the photos?
-genuinely interested