The original assertion that one product in better because they use "premium materials" like "metal" falls into the same trap of "Things made of metal are better than things made of plastic", which is a nonsense schtick that Apple's been selling people for years.
Worth noting that I'm an engineer, and have designed through to manufacturing things made out of both metal and plastic. I'm not unaware of the subtleties.
I just dislike the "Oooooo metal! Must be good!" when it's a lot more complicated than that.
There are no objective truths about metal or plastic as a material for headphones, just what people prefer. I love the metal. But apparently, enjoying my headphones means to some that I've been brainwashed. I find the assertion ludicrous on its face.
You could conceivably make products with such as thing as "premium plastic" (some companies/products do), but the overwhelming experience people have with plastic is that it's often cheap injection molding, breaks easily, doesn't look good, etc.
You could also build crap with metal, so of course there's nothing inherent to those materials.
But the perception isn't entirely wrong.