As pointed out above, it's unlikely that the person who commits the crime is the one that is actually producing the weapons. Thus what the law actually does is make it illegal to own, produce, sell, or distribute guns that would violate the law. Which in turn restricts the supply and makes it harder for a criminal to obtain them.
Without it, you not only have to worry about 3D-print shops mass-producing weapons, but also the possibility of, say, Glock deciding to make and market a polymer/ceramic "undetectable" firearm. Something that, in both cases, would dramatically increase the supply of such weapons on the street and as such, increase the likelihood of them being used in a crime.
Finally, and by your own admission regarding ammunition (BTW, ever heard of ceramics?), even a plastic gun would be better off, not to mention more reliable, with a metal firing pin, metal springs, metal screws, and so on. So the net result is that the law would have no impact whatsoever on the "honest" hobbyist, while at the same time restricting the proliferation of weapons designed solely to defeat existing security systems.