Not usually a fan of articles which are made of of mostly tweets, but in this case I think it's apt. I upgraded to iOS 10 the other day and they of course changed how their music player looked, and I was just used to their old UI.
iTunes sucks for the same reason Adobe Reader sucks.
It's an XML parser. That's the only thing it needs to do to play music files. But then it needed multiple album covers when the ID3 specification didn't allow it, so Apple got haxy. Then it needed to play videos so Apple wrote some more weird XML. Then it did podcasts that updated. Then there was a store. Then it was on Windows: and holy fuck is that a shitshow. Then there were rentals. Then there was the phone... and apps. Then there was a subscription music service. And all of it is underpinned by a poor, benighted little XML parser.
iTunes sucks because it's what a bicycle would look like if you needed to land naval fighter jets on it while also baking ciabatta loafs off the strait of Hormuz.