Ertz is not a great passer. Sullivan isn't great either, but better then Ertz. However, Ertz is better defensively in the 6 role, covers more ground and just makes things happen. (and is good on corners). Also would let Horan and Lavelle (or Demelo) have more freedom going forward. AND Girma and Cooke are better passers out of the back.
USA are missing quite a few players. Besides a healthy Lavelle, also Swanson, Macario, Sauerbrunn (for leadership if nothing else - and to push Ertz into the MF), Sam Mewis, Davidson. Some of them would probably be starters. At least giving the team more depth.
I don't expect them to win the WC. Winning 3 in a row is crazy anyway. But it's fairly wide open this year, sooo anything could happen. But it would surprise me if they even made it to the final.