It is a good thing, people would never be out of a job. Just like many pushed against GSM in Nigeria during its early inception. NITEL workers are afraid they'd lose their livelihood. Some did lose. That was inevitable. But many moved into the new telecommunication companies by undergoing training, a robot would need someone to man it. Some manpower too is required to produce and repair it. The architect may now turn to producing these 3D printers. It would only change the profession of these guys.
3D printing is just a glimpse into the change of the future, just like coal powered trains is replaced by diesel-powered trains. I'm sure some coal expert on trains may be apprehensive of losing his or her job then. Technology can only change, people would have to learn new skills. That is just the worst thing that would happen. Some may lose their job. That's change. After seeing prosthetic leg in action in Nigeria in one prosthetic lab, I can only hope the technology that brought it to bear continues to grow.
Trust me, I get your point, but then not all robots require human supervision. In Asia, there is a factory that is fully automated where robots even create other robots without human presence.
The truth is such a tech would have it's downsides which we barely know of, other than that, it's amazing.
We still have a long way till singularity in robot technology. Until then let us keep our fingers crossed.