If you, for instance, only 3D printed plastic parts you could find in stores, you'd also find that your prints were generally significantly more expensive and less reliable. The upside is the ability to get anything you want at any time, as long as it's sufficiently far from the cutting edge, whether production setup for it exists or not.

The point here would not be to replace traditional manufacturing but to augment it by acting as a backstop for total supply failure.