Hypothetical 18MP D800

Your hypothetical camera doesn't make sense to me; apart from 2fps difference it is just D800, but resampled (or 1fps of a difference - upsampled D700, with lower detail).

Idea of the worthy D700 successor is a D800 body with 16mp D4 sensor, with better color rendition and higher DR at high iso. I would buy such a camera for $3k.

Resampling of D800 images can be done automatically in a preferred point of your workflow, you don't need another camera to do that.
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seb