I didn't think that I alluded to just one medium.
You didn’t make the comment about the purpose of materials. You said in reply to another post “Interesting point.” I made a comment on your comment about the original uninteresting comment.
To use your analogy of the Mona Lisa: Your premise is flawed. If an artist re-purposed the Mona Lisa to make an artwork superior to the original then that would be justified in your view. However if the result was lesser than the original then you would fault the attempt. If the ‘failed attempt’ is still art then it is a subjective opinion that you would use to decide success or not. Whatever the result the result is still art, and whatever kind of art you wish to call it does not and cannot devalue that fact.
To allude to your point I have used as an example an artist you have mentioned and a painting you have mentioned, namely, Marcel Duchamp and the Mona Lisa.
Duchamp made the original of his re-purposing on a Postcard of the Mona Lisa. The right image is a version made for Marcel Duchamp by Francis Picabia. So here we have an image of an image of an image. In the eyes of the conservative art critics of the time, Duchamp’s re-purposing was akin to heresy. But it is still art. Incidentally Duchamp’s wry humour poked a stick at conventional art by adding the moustache and the letters L.H.O.O.Q which are a clever pun. When pronounced in French the letters sound like “Elle a chaud au col” or, ‘She has a hot arse.’ In 1919 this was ‘Rat rod’ art. As the art critic Robert Hughes said, ‘this art was the Shock Of The New’ perhaps this is why more people look at Rat rods than other cars at Car Shows. They may not know much about art, but they know what they like.