While I think suggesting that the 5S is a better phone than the 6/6+ is absurd, I think it's completely valid to suggest that the design is more attractive and conducive to easier use.
I personally am hanging onto my 5S and not jumping to the 6-series. The way I see it is that the 6-series phones aren't necessarily aimed at 5S owners. They are aimed at owners of older 5-series and 4-series phones. Even though carriers are trying to get away from standard 2 year contracts, Apple knows that many customers are either still on them or keep their device(s) for about that long anyway. Screen sizes aside, there really is not a ton of distance between the 5S/6-series phones, in terms of internals. Yes, the 6 has NFC and a barometer, but who knows how widely those will be applicable?
And yes the A8 may be a bit faster than the A7 in terms of launching apps, etc, but the A7 is still 64 bit and I'm not going to complain over taking an extra 1-2 seconds for my next screen to pop up.
The one thing that DID annoy me however, is the fact that VoLTE is restricted to 6-series phones. I suspect that the 5-series phones (or at least the 5S) may actually have the hardware to support it despite not being featured on those devices. I don't know this for sure, but I was sure hoping that iOS 8 was going to bring VoLTE to my 5S.
Of course that didn't happen, but it is moot anyway since AT&T hasn't even rolled VoLTE out where I live at.
The way I see it, every iPhone since the 3GS goes through 4 distinct annual phases in its life cycle, and they are:
First year - New/flagship model (6/6+ right now)
Second year - Modest but very capable (5S right now)
Third year - Entry level/free on contract (5C right now)
Fourth year - Officially retired & no longer sold, but technically still supporting latest iOS even though performance brings it to a crawl (4S right now)
Bring on the 6S/6S+ next year!!