"And it can't stop people from downloading them and manufacturing them with ease at home using a personal 3D printer"

No, that's where your argument falls apart, if you are not allowed to possess a gun then you are not allowed to make one whether you use a 3d printer or a hacksaw, drill and $30 worth of black pipe. Any felon who printed a gun would be treated no differently from any other felon found in possession of a gun.

This ruling was good and important because it affirmed that information and plans are not actually guns and are protected under the first amendment.