But what is the difference between a replica of an item made today, and an item made in the past, if both are made by exactly the same methods?
They are identical - both were made by humans, both are made of the same thing, and by the same method, just one had some time go by. Why does that matter?
'Course, "long term" is still like 50 years away. But I would definitely suggest strongly against investing in antiquities if you're looking to invest on that timescale. This is merely the leading edge of what will become a pervasive problem, and devaluation will probably be ongoing even in the short/medium term. The same 3D printers that sometimes show up on HN will be getting sent to scam artists with motivation to cheat. Only the very high end of the market, populated by people who care enough to take increasingly paranoid security precautions, are going to have anything that holds anything like "value".
(Obviously, I'm speculating.)