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4S had revolutionary and significantly higher CPU capability and new audio processing hardware.
Was it necessary? Questionable. But none of us are the software/hardware engineers behind it to know.

My friend had Siri on his iPhone 4 with a jailbreak, so the hardware of the 4S was not needed to make it work. It's not heavy 3D-rendered stuff you know, just some overlay that records audio, sends it to a server and then gives the result...
 
5S = 5 Sensor
The ads will fit easily with the name and can be easily understood by the populace as the iPhone that has fingerprint sensor technology done right. While if you consider 60fps as the main feature then is something a small piece of buyers would understand to make it worth a buy it now. This will be a 2nd feature and they will just say your photo and videos look now even better or something like that.
 
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