First and foremost it's supposed to be a mobile device. It needs to be portable.Again, the mistake is thinking of it as a phone first and foremost. It isn't a phone first and foremost. It's a hand computer that has a phone... and a camera... and a modem... and and and. The phone is just one amongst many features of the device.
I have no problem with Apple offering bigger phones for those who prefer it. But I find it a bit hypocritical of them to not even offer an up-to-date *choice* for people who favor the current screen size, after they spent years preaching that this is the best size. At the moment it's not a big issue since the iPhone 6 only offers very minor improvements over the 5S, but at one of the next cycles it will fall behind. Also, in case Apple Pay catches on, 5S owners will be left out in the cold due to the lack of NFC and the "secure element". I really hope they will bring an updated 4-inch phone, or at least reduce the exterior dimensions of the 6 in the future by shrinking the bezels.