For myself, I can state with confidence (and I am nearly certain that the same is true for almost all members of this forum) that I am able to fill the frame with most of my shooting, FX or DX, if I so desire. Maybe you need to get closer to your subject or get a longer lens. It is a weird criterion for a camera to assume you're not going to use much of its sensor area anyway.
On a side note, as I am sure you know, FX has an intrinsic advantage over DX with regards to noise because of its much larger sensor size. The D850, with its very high resolution, may be more challenged than other FX bodies here, but if you do the math, the 45.4MP on the 864mm2 of the D850 result in a theoretical pixel size of 19.03um2 (ignoring gaps between pixels), whereas the 370mm2 and 20.9MP of the D500 give you 17.7um2 or about 7 percent smaller pixels. (For FX bodies with lower pixel count, the comparison becomes almost unfair.) I love my DX bodies as much as anybody, but for their crop and resulting reach, not for best noise performance.
Unless Nikon screwed up somewhere, the D850 should perform slightly better with regards to noise than the D500. Since there are a number of variables that make comparative testing difficult, temperature being the most important one, I'll wait for a side-by-side test before drawing conclusions.