Personally, I'm deliberately on the "S" upgrade cycle. I started off with a 3GS, then got a 4S, and now have a 5S. I figure I'll let other people deal with the bleeding edge, and I can get the model with all the refinements afterwards. I'd be willing to bet that a lot of other people think the same way. I don't doubt that the 6 will sell really well, probably better than the 5S, but it may not sell vastly better than the 5S.
But I expect Apple will stick with the C/S distinction for at least one generation, and my next phone will be called a "6S". Or a "6X" for "extended"? No reason they have to stop at two letters/models.
In my case, there was some code dealing with low-level stack walking and such that really needed the actual hardware in hand. Not a common case, to be sure.