I think it was simply not sustainable for Nikon to maintain two complete lines of DSLRs and lenses that were, admit it, pretty close to each other in the grand scheme of things. Sooner or later they had to focus on one DSLR line, and either let the other line die a natural death, or dramatically transforming that line that that it is no longer a near duplicate of the first.
Back around 2002, it looked to me like Nikon had begun to bet the future was with DX, and started focusing on DX bodies and releasing top end lenses in DX. Then, possibly due to success of Canon's FX DSLRs, Nikon decided to better hedged its bets by returning to the FX market and then, with the success of D700, began to emphasize FX over DX.
It makes sense to emphasize FX over DX because DX lens line is not upwards compatible with FX, while FX line is downward compatible with DX. But once Nikon started to focus on FX and let DX benefit by trickle down, DX loses major part of the only big advantage it has over FX, which is compactness. So I think DX DSLR line will just fade into the background.
I personally think Nikon ought to pivot, and begin to think about converting the DX line into a more differentiated product from the FX DSLR line. I think one good future for the DX line would be in the mirrorless world. DX can be a more compact, video optimized, mirrorless line.