Apple's trend is exactly as I described.
When the 5c first came to market, the 4s was still available as the bottom tier phone.
5 was slightly tweaked, wrapped in plastic and called the 5c.
5s was a glossier looking 5 with all the latest internals and features.
It's going to be the same thing this time around, exactly as I described in above.
They didn't wrap the 4s in plastic. They're not going to wrap the 5s in plastic. The s phone never changes. It's the number with no s phone that becomes the c.
A new numbered phone is released. It hangs around for one year, then it gets wrapped in plastic and called the c. The c phone sticks around for two years (like the 5c has done) then gets retired (because it's essentially been around for three years now - one as the number and two as the number with a c attached). By that time the newer numbered phone has been out for a year so then it gets wrapped in plastic, dubbed c, and the whole process continues as expected.
Unless Apple ditches the c approach altogether, that's how the creation of c phones will go. You will never see a s turn into a c because the s is the pinnacle of what they were trying to achieve with that particular make of phone. They wrap the first effort in plastic to try and differentiate it from the superior s model when the s comes to market.
Yes, I see your point. But the 4s was a different size than the 5; even though the 5 was wrapped in plastic against the 4s, it was still bigger and easy to differentiate the two. People had a solid choice: small phone $450 (SMALL!!!! Slower...but $450.), plastic phone $550 (CRAP!!!!), or $650 5s (OOOoooooooh. I'll buy that.) Quite genius.
With this coming September the 5s will be available for *free and we know from the photo that there will be a plastic case for it. (The only unknown is the internals.) They cannot offer the 5s as-is, with a plastic phone the same size with different internals... The differentiator (sp.) for the general public would be the build materials, and not the size.
Hence, in your scenario, I think the thrifty shoppers (which I suspect was the majority of 5c buyers) will simply pick up the 5s as-is, and NOT a plastic 6 the same size as 5s. Therefore, I don't think Apple's decision makers would do that; they want profit.
Encouraging people to either spend $450 on a beautiful, yet older, phone; or $650 for the BEST phone there is doesn't seem to be Apple's MO based on two years ago with the 5c release.
Last year you had 6 (BRAND NEW!!!), 5s (YES!!! I love 4"! And Beautiful!! Save $100!), and 5c (CRAP!!!!!!...but free.... and 8gb.... F.U. Apple... but I'll buy it)
Now, wouldn't it make more sense for Apple to offer the 5s in plastic for $450? It's a two year old phone, and Apple can save some money by putting it in plastic.
(Again, I still don't know what the middle-tier will be, but I am hoping that it's some sort of 6 internals in a 4" frame.
I would be happy if you were right, because a 6 in a 4" plastic shell would be cool. But keeping the 5s as-is will be just too attractive for people deciding between a 5s and plastic 4" 6, and it just doesn't make sense.)