There's very little competence (and also money) required to buy a 3D printer, download a design and print it. A lot less competence than "being a machinist".
The point is that making dangerous things is becoming a lot easier over time.
you can 3-D print parts of a gun but the important parts are still metal which you need to machine. I’m not sure how much easier you just made it … if someone’s making a gun in their basement are you really concerned whether it takes 20 hours or 10?
What you should be really concerned about is when the cost of milling machines comes down, which is happening, quick, make them illegal
The point is that making dangerous things is becoming a lot easier over time.