I suspect this is simply about different kinds of shooters. I do things differently than what you're used to. And I don't think there's anything wrong with that, it just seems odd for you to firmly deny that photographers like me, who handhold D4/5 class cameras, exist.
I do not doubt your experience in your camera club. I'm sure that photographers in your milieu who shoot with heavy rigs use tripods exclusively. And when I go to Wakodahatchee Wetlands, I do see that essentially everyone with really heavy glass shoots with a tripod. I go around Wakodahatchee without a tripod, normally using the 80-400 f/4.5. The combination of the D5 and that lens has enabled me to take pictures I'm really happy with.
I have taken literally hundreds of thousands of photos hand-held with the D4 and D5. My D4 had 450,000 shutter clicks when I switched to the D5, and my D5 has 45,664 shutter clicks. I would say I use a tripod about 1% of the time. My kind of photography is spontaneous and in the moment, and a tripod would slow me down way too much. You could say that I basically shoot like a photojournalist, the subset of photographers for which the D5 was designed and built.
The last 50 of my Facebook albums were taken with the D5, approximately 99% handheld:
David H Dennis | Facebook
A few minutes on Google shows that I am not alone:
Jared Lloyd Photography - handholding with the D5 and 600mm off a rocking kayak! Kudos to this man!
Nikon D5 - talking about walking around, handholding with the D5 at ultra-high ISOs.
Nikon D5 with Sigma 150-600 sport - a bunch of shots he handheld with his D5 and (also very heavy) Sigma 150-600 lens.
Now, I'm not saying that I don't get weary of handholding it after an hour or so of having the giant millstone around my neck, but I do love photography with it and the pictures I take with it.
I just did some reality checking with my favorite pro photographers today. Conclusion: Unless you're built like a gorilla, a 5.5 lb camera for daytime handheld shooting is a no-go. But I knew that already being a strong person myself, where even 3 lbs is too much for long-term use.