I care first and foremost about high ISO performance and autofocus performance. Secondary is having a body with the Nikon 10-pin port as I use it with my Quantum RF remote flash triggers and flash gear and my Bluetooth adapter for syncing with a GPS receiver.
I had the D300 and used it with first a D2x and then a D3 to photograph weddings. The D300 was good up to ISO 1600 and autofocus was the same as that of the D3. Now I have a D800e and a D7100. The D7100 has better low light autofocus than the D800e which in turn is better than the D3/D300. The D7100 produces images at ISO 6400 of people that do not need noise reduction before use.
The box that my old Apple MacPro workstation used was a work of engineering art and this was reflected in its size and weight. Newer workstations have a lot more plastic and are much smaller and weigh a lot less but provide more than 4 time the performance.
At $1100 the D7100 provides far more value than the $1900 I paid for the D300. In addition to the features mentioned the D7100 has two card slots as with the D3 and using the superior (more durable) SD cards. The D7100 has video capture as well as still.
Not to be dismissed either is that I can use SD cards and the exact same batteries in my D800e, D750, and D7100 cameras. No longer do I need two sets of cards and two sets of batteries.
Time to get out of your box. The D750 is twice the camera that the D3s is and at a third the cost. If I wanted the same massive and heavy camera that used two different card types in its two slots and yet another battery that only works in one camera I could spend $6500 on a D4s (and have an inferior autofocus system and 25% lower resolution).
I cannot imagine what wonderfulness would be in a D400 that would be selling for $2000 that would cause someone to buy it instead of the D7200 at $1100 or the D610 at $1500 or the D750 at $2300. Evidently the people at Nikon cannot either.
You don't get why in the
Pro DX Forum there are people wanting a
Pro DX style camera. :-/ Read some threads and you may see what some people want from a
Pro DX camera. They are not all the same, but they are pretty consistent in describing a camera very similar to a D300S with a better sensor.
In the
Pro DX Forum you don't get that there may be some people who don't want to go FX in the
Pro DX Forum. :-/
I'm happy that you have the money, and have found the combination of cameras that suit you. Some people can't afford/don't want multiple cameras, they may need/want one camera that can cover a lot of different Photographic situations. And if any of those situations includes things that are fast moving and far away, a updated D300S type camera is the one that ticks a lot of boxes for some people.
You don't get it, and that is fine, but there have been people on here for years saying what they want from a
Pro DX camera, and whilst some have settled for a D7000/7100, and some more may be swayed by the D7200, and some may have gone FX, or to another brand, there will be some of us in the
Pro DX Forum who may still hanker after something better from Nikon. There are even some that have changed to something else who still say they would buy an updated D300S.
And I'm sorry, but "
using the superior (more durable) SD cards." As someone who has had a SD disintegrate in their fingers, and had another get corrupted, I don't find SD cards durable, and they don't fill me with confidence. They are almost ubiquitous and cheap, I'll give you that, but they are not a superior format, IMHO.
SD cards have been playing catch up to get the performance of the fastest CF cards. Of course if you don't need high performance cards, and are happy with SD cards, then they may be the format for you. Obviously you do have to use whatever format of cards that are in the camera you choose, ;-) but if there is a choice, like there is in the D300S I have, then I will use the CF cards as the primary card every time. Never had a problem with CF cards, and I occasionally need the performance of a fast card.
I try not to say what camera(s) are right for people unless they ask for advice. ;-) I definitely know what is right for me though, and it is not any of the options you offered.

Otherwise I would probably have one or more of them already wouldn't I. :-D