FWIW, I've upgraded annually in the past with each release of the Nexus phone line because each subsequent release was a dramatic upgrade over and took steps to alleviate notable shortcomings of its predecessor. The 6 is the first phone that really didn't have any issues that irritated my delicate electronic sensibilities leaving me thirsting for the next 'big thing' - it didn't miss out on anything I wanted and needed.
I looked, of course, at the 6p, and saw nice things in terms of better still camera performance, slo-mo video, and fingerprint scanner (never used one, but with how much people rave about it I figured I'd enjoy it). But then I consider the downsides (for me): smaller screen, lesser speakers, no OIS, no wireless charging, and Type C USB. I didn't see the merit of gaining features I'd enjoy but haven't been missing at the expense of losing features I enjoy and would miss.
Here I am, nearly exactly 18 months into ownership of my 6, and I have no regrets about keeping it, even amidst the onslaught of some splendid new challengers (S7/S7e, G5, HTC10, etc). There still is no phone out that has the 'killer' must-get feature added that isn't taking off equal value in what it would lose. I'm not saying I wouldn't barter, sell/trade, etc. my N6 for certain other phones out there if it were a net even deal, but I'm certainly don't find anything worth putting more money into at this time as the N6 is still a beast!
I expect I'll likely be looking into either the late 2016 or 2017 releases (as after that point there's no certainty of OS updates for the N6) as they will hopefully bring some real tangible, functional benefits that increase quality of life.
I'm intrigued by Project Ara if it really gets off the ground (mostly for the possibility of niche modules that could be added (modules that allowed adding a real camera lens/sensor (it would stick out a bit, but would be removable and a better alternative than lugging an independent camera when normal premium smartphone performance won't do); module that stores bluetooth earbuds; module that integrates cardreader/usb key; module with tip scuffer for your pool cue; imagination is the limit...). I'm honestly not too optimistic that the first generation (being the first phone Google will ever have truly built in-house, and on such a radically different design curve) will be a great device, but I'm not ruling anything out - and if nothing else, it may inspire some truly great works from the status quo.