3D MCAD is not “done.” Not by any means.
Well over 40 years after the introduction of commercial 3D MCAD products, and 25 years after the introduction of feature-based solid modeling MCAD, many MCAD programs aren’t totally 3D. They’re 2D/3D.
What I mean by this is that their 3D capabilities are inadequate to represent real world products fully in 3D. They have to rely on 2D drawings as a crutch, because their 3D annotation tools (GD&T and PMI) are either weak, or missing entirely.
In short, with many CAD systems, 2D is the master. Not 3D.
Recently, Dassault Systemes posted an excellent video showing the benefits of using a 3D master approach. I think it provides a good starting point for the discussion.