I upgrade every 2 years or so, so I too will wait until this fall to upgrade from my 6. Then hand my phone down to a family member using an older model.
If I upgrade now, I'm moving to a more frequent upgrade cycle, and that costs money
New phones add new LTE bands, possibly improving coverage and speed, depending on carrier.
Maybe I'll go with a different form factor. Every iPhone I've had has had some screen improvement from the previous. The 4 had retina. The 5 had a bigger screen. The 6 yet another bigger screen. Do I go crazy next time and increase size again?