Why an upgrade
I usually upgrade to new iteration of same camera mainly for better battery and screen resolution. But upgrade my system when equipment is limiting my shooting style. Upgraded to premium point and shoot when the focus speed and low light capture was limiting my first Canon. Upgraded to m4/3 when LX series didn’t produce good 16″ x 20″ prints. I tried Nikon APS-C sensor, but never felt the images are that much significantly better to warrant an upgrade.
I never imagined possibility of 60″ x 45″ prints from single file, but the D810 and the Hi-Res mode changed my mind.