” I can envision a world where whatever we want will be sold for 99 cents, just like an app.”
That world is well into the future, At this point in time, most 3-D printers can only print one material at a time, or only one material (ink) period - usually a plastic. If you move to printers that are capable of a wider range of materials - say metals or glass, then the costs skyrocket as do the economic limitations for their adoption. The market for 3-D printing regardless of the constant flow of uninformed media hype is still almost exclusively limited plastic prototyping. It isn’t going to threaten Chinese manufacturing, it isn’t going to create weapons, it isn’t going to build anything that is multi-material and complex - or compete with existing cheaper and faster mass manufacturing processes.
Arcam AB, a Swedish company makes electron beam 3D printers that use titanium. It's not all plastic.