The D5 is a wonderful camera in low light. But, so is the Nikon D3s.
I had the opportunity to shoot the D5 last year in Florida wildlife preserves, I also shot with the D850 and D500. I will be honest and say I let the D5 go because I was also getting great results from my D850 and D500 in low light situations.
The D5 shot faster, but the extra 3.5 frames per second was not enough to justify the cost difference. With the D850 and D500, I still had enough money left over to get the new 500 5.6.
However, the shot below was taken with the D5 with the 70-200 2.8 FL lens. It is an image of a adult Sand Hill Crane feeding it's colt a dragon fly. The original image was cropped about 60%. I could have been using the D850 to get the same shot.
If your a professional shooter who can easily afford the D5, I say go ahead. But to be honest, I do not miss it.
If you like the shot, it is one of my featured shots in my photo show.
My photo show will be at the Weller Gallery at the Davis Education and Visitors Center in Fellows Riverside Gardens, Youngstown Ohio. 123 McKinley Ave. Tuesday-Sunday 10-5. Closed Monday. 330-740-7116. The show will run from August 17th to October 13th. Meet the artist date is Sunday, September 8, 1-3 pm. See you there.

The Nikon announcement is that it "is developing" the D6. I do remember Nikon announcing that they were developing a camera that they then decided to scrap (forget which one it was but they never finished the development). But it is likely a real announcement will be forthcoming with price and details, so whether you call this a rumor or not is irrelevant. However it is a game that companies play, trying to keep people interested in their products so they don't jump to another brand (ie. Sony). However it can backfire, people interested in a D5 may hold off on their purchase to see about the new D6.