He did get a character arc. If you watch the Zoro vs Killer fight, there were less grey cubes flying on the screen and less colors, but then when he fought King there were a lot of grey cubes and a lot more colors. Then he said he King of Hell, before he was not king of hell now he king of hell so no king of hell then king of hell, that's an arc. Other lesser characters require an entire dedicated story to go from hick to King but there is no character as great as Zoro in fiction, like Mr. Morj told me, so he did it in a few chapters. Also more black squiggles. Lanji cried a bit then kicked a fat guy, that's his character arc.