Carvalho is a lot more physical and mobile then Carrick, pretty much, along with being arguably better defensively from what I've heard. Carrick and Kroos would both be way too stationary and are very similar players. But then again Scholes-Carrick worked, but that was 5-6 years ago. Carrick of these days is too static and is the one dictating tempo, and Kroos is just a much better and more creative version of him. Not quite as good defensively, but he's not shit defensively by any means. And yes, Bender is great defensively, but Sahin is the deep lying playmaker for them and it'd essentially be the same formation and tactic as playing Carvalho-Kroos. One defensive midfielder who is very good defensively, but also a mobile player, next to a more creative player who dictates the tempo. Gundogan is different in that he takes more risks and is quicker, runs with the ball more so I'd prefer a player like him, but there's no reason as to why it wouldn't work with Kroos.