CNC machines will probably continue to be the king of things they can machine. The catch, though, is that you can 3d print a lot of structures you cannot machine by any means (see: SpaceX SLS's their rocket nozzles).
You have to design for the manufacturing process. Parts designed for CNC machining will always feel like a square peg in a round hole for 3d printing. Once you start designing for 3d printing, though, things get more competitive quickly. You can design parts to have hard to machine shapes, or parts that would have too much waste to machine from a solid billet of material and 3d print them without the waste.
Your computer case is bent up from sheet metal instead of machined from a solid block of steel for a lot of obvious reasons, and it makes no sense to compare the processes for a design like that. 3d printing will enable similar shifts of design.
