Even if it comes to pass that metal 3D printers become cheap enough for people to start whipping up high quality guns at home, people will still be willing to pay for new and innovative products.
I suspect that you do not have much personal contact with the American gun culture. I am deeply immersed in it. I see guys selling their Third generation Glocks to buy Fourth generation versions of the same model.
Despite the fact that the 1911 is old and proven technology that nearly any manufacture can copy, people are will willing to pay more money for a Kimber because they make a really high end product.
If you're going to argue that eventually 3D printing will be able to produce pieces that are identical to those that can be made by high end manufacturers, how can any law be enforced against them? Once they're out of the printer, there's no way to tell where it came from.
LK