Yes, I think I agree that some degree of novelty is required. (Reductio ad absurdum: A robot that generates copies of Beethoven's symphonies -- by some fancy process that doesn't explicitly involve copying them, but is none the less guaranteed never to generate anything Beethoven didn't already write -- is generating first-rate music but is also absolutely useless and would not be regarded as any kind of artist.)

But I suggest that the degree of novelty required is fairly small. How valuable would it be to the world if someone were able to write another nine symphonies just as good as Beethoven's but that don't enlarge our understanding of music any more than if, say, Beethoven had been able to work twice as fast and lived slightly longer, and had written them as his numbers 1.5, 2.5, ..., 9.5? Pretty damn valuable, I think. If someone found the manuscripts of nine other symphonies Beethoven actually did write but for some reason never released to the world, it would be very exciting and I bet there'd be no shortage of performances and audiences. Or: Pick one of Haydn's 104 symphonies. It probably doesn't really enlarge the world of music much beyond the other 103, but the world would definitely be poorer for its absence.