only the non critical to function parts can be plastic

This just isn’t reality. For .22lr, nearly 100% of the gun can be plastic, including the barrel. Here are two real examples where the only required metal component is the firing pin:

cnet.com/…/the-3d-printed-gun-controversy-everyth…

3dprint.com/…/worlds-1st-3d-printed-revolver/

For more reliable and more powerful guns, some critical components must be metal, of course, like the barrel might at least require a liner, but the majority of the gun and internal mechanism can still be plastic, not only non-functional or cosmetic parts.

And you can use literally anything else other than 3D printed stuff. So why is 3D printed even an issue.

Idk, I didn’t say it is. I’m just informing you that 3D printed guns are real, not a “fallacy”, some function very well and reliably, and can actually be made with almost no or minimal metal parts