Casemiro is a better fit for the single deepest holding mid of a 4-3-3 than De Jong is. It's just a surprise that it has taken until now to go after such a player.
Based on what we have seen from Ten Hag so far, in a positional sense, Casemiro is a better fit than De Jong, but we still need to sign a deep-lying midfield playmaker too. That said, Casemiro alone in the side should mean that Eriksen and Donny could both play more of a natural game though with Casemiro holding, and even Fred and McT can do what they do best alongside him.