Disclaimer: I have only hobbyists knowledge of CAD and 3d printing
The author may be a mechanical engineer, but he certainly isn't an entrepreneur. The interesting question is not "Does it make sense for everyone to have a 3D printer now?" but "What kinds of things become possible once everyone has a 3D printer in their home?"
You are putting the cart before the horses. The first question is "why would anyone buy it?", and if you don't give an excellent reason for that, you will not see a 3D printer in everyone's home. And this makes your question about possible things as useless as a designer's contest (where most of the prizes are awarded for physically impossible designs).
The author may be a mechanical engineer, but he certainly isn't an entrepreneur. The interesting question is not "Does it make sense for everyone to have a 3D printer now?" but "What kinds of things become possible once everyone has a 3D printer in their home?"