I question whether Rowling should be on the list at all, when The Last Unicorn is not.

I would say the same thing about the absence of Engine Summer (or Little, Big; YMMV) and . . . well, every other book on the list.

In terms of unity of narrative arc, Harry Potter needs to be included in its entirety more than Earthsea and (as I understand it) way more than Discworld. I would also opine that the third volume is easily the best (intersection of increasing skill and decreasing editorial control).