Why if it's in a 1 series style body couldn't an upcoming high MP body be called the 1Ds mk IV?
The 1D X has most of the resolution of the 1Ds mk III, and other than reach, everything the 1D mk IV had - while it wasn't a direct descendant of either, it made it a suitable replacement for both lines, even if a few missed either the resolution or reach. Having said that, putting FPS before MP in the 1D X is more of a trait of the 1D series than 1Ds - it's just the change in sensor size which necessitated a new naming convention.
The 1Ds line were high MP studio cameras. And as proved by the G12, G1 X and then G15, the new X variant doesn't necessarily mean the end of the line for the old way of doing things.
Calling it the Canon 1Ds mk IV makes more sense to me than calling it the Canon 3D