To me it seems that so called "full frame" ( like D4 and D800 etc.) kind of sits in a sweet spot of size, ergonomics and price. At least every time somebody complains about the lack of quality in APS-C or smaller sensors they compare it to FF, not mid format cameras. So why does FF always give enough IQ (mid format is hardly ever mentioned in this context) and smaller sensors not, even though modern APS-C cameras are better than FF bodies were 6 years ago and smaller sensors are better than APS-C was a few years ago? FF is the gold standard and we are reluctant to admit that APS-C, or even M4/3 is now good enough for most purposes, even professional photojournalism. After all I liked to shoot with the ancient EOS-1D with 4.7 MPix, as it gave better results than Provia, so just about any serous small sensor camera should be good enough for me. There is also, for professionals, a distinct lack of believable true pro quality systems with smaller sensors, systems which could seriously challenge the pro lines of FF cameras from Nikon and Canon. For one thing they would be DSLRs, not mirrorless, as the EVF time lag is not acceptable in many situations. This is mostly a lens problem, making a slightly smaller pro body would be easy, but building a large collection of lenses is hugely expensive. Nikon and Canon have a 60 year history of 135 format lenses to build on.
That said, if I made any sense at all, I shoot mostly with D4 (speed, ergonomics) and D800e (quality, quality) even if (large) part of the IQ goes to waste in print. On the other hand I have a full Fuji X-Pro1/X100s kit with lenses from 14 to 200mm and like them a lot for travel and reportage for their size, weight and "camouflage" quality (harmless old man with old camera). And the IQ is, again, more than good enough, and also slightly different in a nice way, even others notice it. So there is room for all systems (except mid format for me) depending on situation. As long as we have hands and arm strength like we have now something like D4/D800 feels OK and provides a good "standard" for lesser formats to aspire to. In IQ they are almost there or close enough, from the usability and ergonomics side of things going too small is not good, if we are talking about tools, not something to snap a vacation picture with. Many times I take D4 to an assignment, just because it is bigger than D800 and has an integrated vertical grip...