It is definitely possible for a good modern painter to forge a "masterpiece" that art lovers and experts will attribute to a famous artist. For instance a "Supper at Emmaus" was created in 1937, and hailed by some of the world’s foremost art experts as the finest Vermeer they had ever seen. For many years this forgery must have given to art lovers the kind of pleasure that they enjoy when admiring the original paintings of the Dutch master.
Could a musical "masterpiece" attributable to a famous musician be composed today? For instance could a modern composer create a complete Beethoven Symphony No. 10 (forget the fragments assembled by Cooper). The ground rules for this exercise are:
(1) It would sound like a great Beethoven composition to a music lover. In fact Beethoven lovers would enjoy it as much as they enjoy their favorite Symphony (No. 7 for me).
(2) It would be original, its themes would not be copied from existing compositions.
Why we don't have a "Symphony No. 10 " created in modern times? It would add 10% to the pleasure that we get listening to Symphonies 1-9. I would definitely buy a recording, and enjoy it in a concert hall.
(A) A talented composer wants to be original. But most music lovers don't care for contemporary "original" classical music. Concert houses would quickly go bankrupt if their repertoires were based on contemporary music. I expect posters of this board to strongly disagree, but that's the real world.
(B) The space of original tunes is limited. That's way we have Atonality and Serialism. The "Symphony No. 10" would have to copy existing tunes and themes.