Nice mock ups pics! The thing that got me going on about the concept (which I had seen spoken of in older posts and discussion on why a module may or may not be favored as a production effort by DSI) - was when I popped the hood on the synth and observed that the control panel is conveniently separated into two sections. The entire zone to from the left side of the display to the right edge of the control surface is one PCB, everything on the left of that line is another, they're connected with a ribbon cable. This is the largest single sub-assembly in the unit, (besides the keybed) at about 16" wide. The power supply module is actually pretty large, possible a desktop module could use a smaller type? The voice board with all the jacks is much smaller than the knobs board, and neither of those are as wide as the Rev2 Desktop at 21.6" (that probably includes the end cheeks).
So it almost looks like with very little re-design a desktop box could be achieved by basically arranging existing components and designing the enclosure. Likely there would need to be some mode switching to get at the parameters that would not have dedicated knobs - but that seems like a manageable issue.
All that said - would bet that the choice not to pursue it thusfar likely comes down to a market evaluation - even with minimal dev effort necessary it would constitute another inventory item and would pull resources from other angles to support the inevitable peculiarities of modifying the OS.
I do wonder if it's generally assumed, (and if so would challenge the idea), that a desktop module inherently needs to considerably cheaper than a full keyboard. (Guessing that historically is what is assumed) - but there are certainly use cases for a smaller unit that isn't simply a stripped down version of the full instrument, rather some kind of extension to it.