I wouldn't be so sure. The 4S sold over four million units on its opening weekend, which is pretty close to the 5's five million, but way more than the 4's 1.7 million.
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.
Yeah, I'm on the non-S cycle, going from a 3G to a 4 to a 5. Each of those had a feature (3G, Retina, LTE) I wouldn't have wanted to live without for a year, but for quality of experience and useful lifetime, the "S" cycle is the place to be.
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.
I think it's also a good idea to wait a month or so after release. What's the rush? Let the first production runs go through and work out the kinks. So far I have heard some reports of screen issues on some 5s models. With a new and unusual part like the fingerprint sensor I'd rather wait a bit.
Personally, my rush was that I needed a device with an A7 in it for testing purposes. If it wasn't for that, I'd be right there with you and would have just ordered online and waited.
It will be for almost everybody, as long as you have someone somewhere with a 5S to use for a sanity check before you ship.
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.
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.