I imagine you're correct that if a theoretical perfect duplicator existed, the physical art market would be destroyed, but only because there would be no way of authenticating the original. The original still has value if you could prove it, but nobody would know which one the original is.
Let's say this theoretical duplicator was perfect in every way EXCEPT you could tell it wasn't the original because the isotopic signatures didn't match. That is, for all intents and purposes, each duplicate is exactly the same but each instance encodes an unreproducible signature that can be used to identify it.
I postulate the original would still carry an enormous value relative to the exact duplicates.
Let's say this theoretical duplicator was perfect in every way EXCEPT you could tell it wasn't the original because the isotopic signatures didn't match. That is, for all intents and purposes, each duplicate is exactly the same but each instance encodes an unreproducible signature that can be used to identify it.
I postulate the original would still carry an enormous value relative to the exact duplicates.