"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.
How would they know if you did it?
are we assuming they are not monitoring your internet and credit card activity? If we assume they are not doing that they can bust you when you use it or while transporting it, like they do any other person who is possessing a gun they are not allowed. If you leave it at home in a drawer, and don't commit any other crimes that cause your house to be raided, then you should be fine. If you have a clean record and you can legally own a gun then you can legally make any gun you are allowed to own. It would be very illegal to try to sell it without a manufacturers permit.
Certainly selling it wouldn't be the point as anyone can make one of their own. The government now and soon will have no idea how many 3D printed firearms are in people's houses. They don't have the resources to effectively monitor every person's Internet for a questionable download. There are also secure means of transmitting information over the Internet. Plans can be passed along by hand on USB sticks as well. This development certainly gives the civilian populace more teeth against a would be despotic government.
In fact you can buy a desktop CNC milling machine and all of the files on a USB drive to finish 80% receivers.
assuming they could get bullets and all the other parts needed to make guns, we can in the US but that is only because we don't currently have a despot in charge.
It was already fairly easy to finish 80% receivers and make black pipe shotguns and I assume that this will be just as popular as that. You can also make an AK receiver by printing out a template that you glue on an appropriate piece of square stock and then drill out with a drill press.