Where's the evidence that the iPhone 6S is going to undersell the 6, especially when we don't even know exactly what the 6S will bring? There are certain things that seem likely (A9 chip, Force Touch, better camera, more base storage), but we don't know just what they'll be like or whether or not they'll lure customers in. You can't use "I have a hunch it'll fail" as a reason, especially if you're a known Samsung cheerleader.
Remember, the iPhone 5S still helped Apple's sales despite being a pretty conservative update to a tiny phone. This may not be a revolution, but it's still an improvement on larger phones that have already been wildly successful. Remember, Apple's sales patterns are partly dependent on upgrade cycles -- there are a lot of people using 5s and earlier devices who've been waiting to leap to bigger devices. You also have to figure in rapidly expanding sales in countries like China.
I'm not expecting a 6-level increase, but nor is it going to crash and burn. The question is whether or not other manufacturers will see healthy improvements of their own. I can see Chinese OEMs thriving in the near future, but Samsung/HTC/Sony and the like? Not so much.