To be honest I think that 20 years from now people won't look back and say "3D printing, ahah! that was the revolutionary invention" but it will be a part of a larger set of inventions and innovations which will revolutionize manufacturing.
For example, I think 3D printing as a method to rapidly make metal casting and injection molds (either directly printing the molds or via lost-material casting) is potentially more impactful to manufacturing than direct 3D printing. The truly big innovation of 3D printing and, say, "table top manufacturing / fabrication" systems won't come because such devices will be immanently practical for home use. Instead what I think is more likely is that the ability of hobbyists and tinkerers to experiment with them will lead to an explosion of innovation. Meaning: better tools and techniques for automating and simplifying the process of transitioning from design to manufacturing.
For example, I think 3D printing as a method to rapidly make metal casting and injection molds (either directly printing the molds or via lost-material casting) is potentially more impactful to manufacturing than direct 3D printing. The truly big innovation of 3D printing and, say, "table top manufacturing / fabrication" systems won't come because such devices will be immanently practical for home use. Instead what I think is more likely is that the ability of hobbyists and tinkerers to experiment with them will lead to an explosion of innovation. Meaning: better tools and techniques for automating and simplifying the process of transitioning from design to manufacturing.