While I've lusted for a Leica Monochrome for some time now, it's not really financially feasible. I just muddle through with what I have and make the best of it. I also don't own one of the newer mirrorless Nikons--my newest model is a D810.
Comparing B&W results with the D810 and the D700, it's like medium format vs 35mm film to in my estimation. The higher resolution and sharpness of the D810 sort of overpowers the emotional impact of the "gnarly" in the D700. I just like the older files more for B&W, just as I preferred the look of 35mm film to medium and large format B&W in my youth. For instance, I like Jeanloupe Sieff's Death Valley photos more than Ansel Adams' landscapes. Personal preferences overwhelm reason, I guess.
I also use the D300, D2X and D2Xs models in DX format for B&W. I like the results a lot. The older models seemed to be able to produce technically less perfect images but the photos had more soul if that's even understandable. Again, just my weird viewpoint.