Art? I haven’t looked into it much, but I don’t really know of any significant improvement in fine arts for a very long time—not in style/technique and not even in the technology (e.g., methods of casting a bronze sculpture). I’d also suggest that music has gotten less sophisticated, but this is super-subjective and treads in culture-war territory, so I’m just going to throw it out there as a wild-ass hypothesis for someone to follow up on at some point.
I’m a little bit late to the party here, but there are examples of improvements in sculpture technology/technique/style leading to new (& very beautiful) works of art, see e.g. Barry X Ball’s works made with a combination of 3d-scanning, CAD software, CNC mills & traditional techniques. Not to mention he has a wide variety of stone available to him thanks to the global trade system.
As for music, I guess that totally depends on what you’re comparing. The proper comparison for today’s popular music isn’t Beethoven or Bach but folk music & perhaps music for drawing rooms & salons, which, although they had their own beauties, were nowhere near as complex & intricate as the traditional European art music that is most listened to today. Of the past, only the best survives, but in the present the good & the bad coexist. That said, I think maybe there’s a kernel of truth in what you suggest. But we shouldn’t trust our intuitive judgment on this.