You know, i was very happy with my 5D3, except high ISO's in low light. When the 5D4 was announced i paid it no attention at all. It was having the 80D and loving the new features that made deciding which camera to take out a problem. Sharper images from the 5D3, but a much more fun camera in the 80D, and IQ more than good enough. I did crack a couple of months ago though, but I'm one of those people that just has to do it and cant wait.
Yes, its an upgrade, and a good one, but for me its more the smaller changes that make the difference. I love the new AWB that the 80D has, and auto ISO with EC is another 80D feature i wanted. Its like a 80D on steroids
I still find i have more fun with the 80D though, and i expect ill use that more than the 5D4, but when the situations demand FF the choice is now a lot easier to make.