Can plastic 3D guns be made, yes. Are they actually durable and have the potential to replace metal firearms, no. Can metal printed guns replace conventional firearms, not likely. There are limits to the process, and what is left out of the discussion is whether or not the public would buy one.
Are polymer AR15 lowers a significant part of the market now? No - if a plastic AR lower isn't common yet, it goes to the public not seeing them as worth the money - even if they are substantially less expensive. Many are used in .22 kits because of the lack of reputable durability. Few trust them for long term use in anything bigger.
There's also the issue of being able to make a gun from existing parts shipped directly to the home more cheaply. Other than the serial numbered receiver, you can build a gun at home in nearly any configuration you want, however highly illegal it may be. If you buy an unfinished receiver, no one would be the wiser until you flashed it around to get attention from the BATF.
The reality is that criminals really don't give a flip about printing guns on the sly - they circumvent the entire process of learning a programming language, buying the equipment, and perfecting the process by a very simple technique. They steal them. From you and me.
The 3D printing "horror" is just a political gambit to restrict the rights of lawful gun owners assembling their own guns from parts at home. First, the legislature will outlaw printing them, but the language can and will be subtly altered to make it illegal to simply assemble them from parts however legally obtained. There is plenty of existing precedence.
It took almost 30 years for the Star Trek communicator to become the cellular flip phone of today. 3D printing isn't going to happen very soon, certainly not at the homeowner level, and certainly not in a way that will create a firearm that is as capable as the ones we own. You can't rewrite the laws of physics - we don't have plastic handgun barrels yet, if the makers could have they would have. It's all about profit to them.
To the politicians, it's about control, and that's why they are all jumping on the 3D gun printing issue to "keep them out of the hands of criminals." Same reason Obama ordered that we couldn't buy Garands from Korea - identical to the ones sold by the CMP.