As was pointed out 3D printed guns are at this time are primitive. The materials available for 3D printing are not sufficiently strong to produce a durable weapon - example. Even if quality materials were available, the high end 3d printers that could use them are extremely expensive (tens of thousands of dollars) and sold/leased by a very limited number of companies, and as such it would be easy to trace who has those printers and keep tabs on them. Additionally you won't be able to get materials from just anywhere, and government regulation could keep track of orders for materials that could be used for such purposes. There's also again the matter that 3d printing will require technical expertise that most people just won't be able to get.
I generally agree with your points except for the last. It would require that expertise to come up with the plans in the first place, but once developed and released, they'd be easy for anyone to use. That's one of the attractions of 3D printing after all.
