You seem to be misinformed about how criminals get guns. Here’s a hint, they steal them, buy stolen weapons, or have a straw purchaser buy them.
Also, the vast majority of NICS rejections are false positives.
Those 2 million criminals are still going to get the guns. They’re not going to be 3d printed ones, they’re still going to be awful $200 Lorcin’s, Jennings’, and High Points.
If you want to be concerned, look past the US to the rest of the world. The place that these could really have an impact is in Europe and Asia.