with low light performance of the 5n, what is the appeal of full-frame?

I'm not confusing facts with psychology.

I am fully aware of (and covet) the factual advantages of FF sensors over smaller sensors.

• The fact that other things being equal a larger pitch pixel wil collect more light and have less noise is well known. 
• The fact that larger sensors allow a shallower depth of field covering the same FOV as a smaller sensor is also well known.

Except for size, I thought the D700 to be the perfect camera at the time it came out. I would prefer the latest generation technology be applied to a full frame sensor with a lower pixel count, since I shoot in the dark, and I've always been an available light photographer.

BUT the question that was asked was about what explains the  appeal of FF cameras. And I think FF, for its benefits, has an appeal that exceeds its indisputable benefits. So my answer addresses the gap between the real benefits of FF cameras and their appeal. Psychology does play a role here.