Well, Trevor, if you've been shooting a D700 until now Id say you deserve a new camera. As far as convincing your wife of the wisdom of the purchase, I hope you pointed out the economics.
The D700 was released in 2008 at the same list price as the new D850. Over those ten years inflation in Canada has averaged about 1.5 percent. So in adjusted dollars the D850 only cost about 60 percent of what the D700 did. On top of that it has four times the resolution. 60 divide by 4 is 15 so considering cost per pixel you really only paid 15 percent for the D850 of what the D700 cost. Current pricing for used D700s run about 10-20 percent of original. So you can sell the D700 and the net result is that the D850 was essentially a free upgrade. So when it comes down to it buying the new camera is likely one of the best financial decisions one could make. It's perfectly logical.
Just a word of caution. I ran this argument by my bride. If you wife's method of reasoning is similar it falls apart at the mention of how much you paid for the D700 ten years ago