Lets put this whole thing in perspective:
1) Overall sales for everything and anything are down. All the recent news stories were that overall, "Black Friday" sales declined by 11 percent this year. In direct contrast, some people had been predicting (wishing for ?) a 3 to 4 % increase;
2) Big chain stores like Sears, JC Penny, Radio Shack, etc, are all facing big trouble, and are looking at closing stores, if even surviving at all in the long run;
3) I dunno about the rest of you, but they just closed another major factory here in my hometown putting hundreds out of work. This company had been here almost 100 years, and there is no replacement jobs for those people. You think any of them have money for a DSLR?
4) Although I love my D800, I still use my D700 all the time. It works, it is bullet proof, and most images today are going on computer screens and smart phones, not billboards. For me, there is NO economic reason to throw away my D700 and get a second D800 or a D750 or a Df, etc. None what so ever. It saves me money too because I use an older computer (clower, smaller hard drive) than I have to use for my D800. Your mileage can and will vary, but frankly, I need my money for other things that just buying an upgrade for the sake of upgrading by itself;
5) The trend seems to be using a smart phone for everything. Messaging & texting, note taking and calendar planning, photography, replacing a credit card, watching movies and TV shows, on and on and on. It is like society is being herded towards everyone having a smart phone and using that device for everything in your life. So it is NOT just DSLR sales that are declining due to smart phone use and trends.
If you look at the whole, big picture, all factors together, why is anyone surprised that DSLR sales might be declining?