The 1DsII has the fast EOS-1 series focus which is indispensable to action/sports/events and snapshot shooters. The 5DII cannot really push the superteles that fast. SO there's no replacement for the 1DsIII.
True, if we expect the D700X to be of a higher grade on the non-digital side than the 5DMkII is. But I suspect that most action/sports/event shooters go for the 1DMkIII and D3 rather than the 1DsMkIII and D3X.
Also, the 1DsIII will just bounce off the floor if you drop it. This too is a form of insurance for pros. The 5DII is not as tough as it should be as it seems to have had a 25% failure rate on a certain Antarctic once-in-a-lifetime trip.
The D3X will probably have that sort of advantage over the D700X too, though again, Nikon might spec. the D700X better than the 5DMkII. (Which is part of why I expect it to cost more like $3.5K.)
So on your first two points, your main reason for seeing the 1DsMkIII in a better position that the D3X is that the 5DMkII is inferior to what you expect of the D700X. If that is so, what the D700X causes Nikon to lose in D3X sales, it should more than make up by taking sales from the 5DMkII from those who care about things like AF and build quality.
Fast focus, DR, good high-Iso, video ability, vibration reduction seem to be things people want from top line still cameras, in addition to lots of pixels or a large sensor. Some want compatibility with legacy lenses -as offered by the Nikons.
That is a fair list, if we add high frame rate, and the build quality that you mention above.
It might be that compatibility with a substantial investment in Nikon lenses is a major reason for people buying a D3X rather than any of the less expensive 35mmFF options, for now. I can see the D3X and 1DsMkIII both selling far less than the 1Ds series used to ... but still not in such small numbers as some other high priced niche products that survive, like the Leica M8.