It might be easier to write CAD software without an OS. At least, writing directly to a framebuffer is simpler than using X11 or OpenGL or Win32. Unless I'm missing something, a modern CAD system has to plot each entity twice; once to the display system and once to an internal grid. The internal grid is used for entity selection, possibly culling and other things.
The older AutoCADs (on DOS) apparently plotted the internal grid (hi-res), then resampled it to generate the screen display.
Thus, minor panning and zooming could be accomodated without "regenerating" -- replotting the hi-res grid.
Anyway, a CAD system doesn't really use the facilities of an OS; something like a web server, however, seems to benefit from an OS.
I'm not sure how this fits into the discussion, but what you're saying can be used as an argument against IP.
Since people lived just fine before technology, we don't need technology all that much, therefore we don't need IP.
Edit: For example, could you write CAD software without an OS as we know it today? It would be undoubtedly harder, but you could. People did this in the 60s.