>> Pixel density is the nub of the whole issue as this impinges on high ISO noise
I think the D800 demonstrates that that is not the case. It has about the same low noise performance as the D4 at the higher ISO's and it apparently has far more dynamic range at the lowest ISO's.
The end game for pixel density is more a matter of diffraction. If we look at the smaller sensor formats we see far smaller pixel sizes than in the DLSRs. The difference is that small formats rely on shorter focal lengths and therefore arguably don't need smaller apertures to achieve decent depth of field like the larger sensors need. There is also the issue of... do all those pixels make sense in a P&S camera, for example. But that is a market psychology issue, not an engineering issue
The end of the pixel density war has a lot to do with the tolerance for shooting at wider and wider apertures to make use of those pixels. But that also is not an engineering issue, it is really a market psychology issue, and that is why I said in my last post that I don't want to be the one to declare the end of the pixel wars. Market psychology is too strange for me to have the temerity to do that .
Edit: what I said here about the D800 implies that there may be some benefit to more pixels that have nothing to do with resolution and if that is the case then there may be no conceivable end to the megapixel race!