Aesthetically, the 5 was already not that much different from the 4/4S. Yes, the manufacturing process was quite different, but the look felt similar. More of a facelift than a redesign. Keeping the same design for the 5s just makes it feel even more dated.
For this industry, I would say minor changes (e.g. 2G -> 3G; 4 -> 5; iPad 1 -> 2+) every year, and major changes (3G -> 4; iPad -> iPad 5 / Mini) every other year would be reasonable. Even those "major" changes I listed above are still less dramatic than what you see in Android phones; from the front, the iPhone's look hasn't changed at all asides for the removal of that chrome frame.
In the iPhone's case, the only aesthetic change that wowed me was the upgrade to the iPhone 4. It just screamed "high end" and was a fairly radical from traditional smartphone designs. Of course we later found out that they were inspired by Sony's aesthetic (which is probably why I like the look of the XZ1... if they make a "mini" version of that, I'm buying it)
Regarding 5 vs 5c: I would consider going from metal to plastic to be a fairly significant downgrade. Especially when the plastic is only available in childish colors. It looks like a fisher price toy. They should have modeled it after the iPod touch 5. I'm not sure what improvements you're referring to. I didn't see anything mentioned that the 5 didn't have