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I built one (actually 3.....it took a while to get it figured out, but only kept one and destroyed my first two abominations) Polymer80 lower. It's serviceable. The issue with regulation is that the CNC technology to build an AR, or any small metallic item, is rapidly becoming widespread. Look at the Model 70 pre-'64, the Model 70 post-'64, the Model 70 Classic and the Remington 700. The Model 70 became the post-'64 because the huge line of machine tools and skilled laborers to man them became too costly. The post-'64 70 and Remington 700 addressed this with design for manufacturability. The Model 70 Classic took advantage of CNC machines becoming cheap enough on an industrial scale to allow for complex receivers and bolts to be milled again.

That process didn't end in 1990 with $500k 6 axis machines the size of a shipping container. The electronics were the secret sauce, and their cost has plummeted. The cost of a sufficiently accurate machining setup has also fallen some, but in aggregate the cost is now a tiny fraction of what it was. The fracture toughness of 3D printing is abysmal, but the idea of a cheap, widely available device that turns a CAD file into a 3D object is now common knowledge.

Security through obscurity has rarely worked, and in the case of firearms, it is currently in the process of failing. People will respond to that change by following their prejudices: some will try to regulate chunks of metal while others will continue to insist that we address the root causes of people killing one another.