Kroos is playing in Madrid in a 3 men midfield position using Casemiro to protect the back four and allowing him and Modric to push over. There's nothing suggesting that he'll be able to handle a 2 men midfield with only. The same problem will occur, Matic will be left isolated at the back and Kroos will need to drop back to support, preventing him from going forward and his defensive abilities aren't even that good anyway. Midfield will look even worse.
Considering the fact he's approaching his 30s and his decline physically will happen sooner or later, I can only see one winner of this deal actually. Madrid will get a 24 years old midfielder, play him in his natural position in 4-3-3 and go on and win CL again (Juve were very close to win CL with Pogba in part of midfield 3), while we will get stuck with the same problem we had previously, trying to push a midfielder in a position that does him no favor and expose the team defensively, only difference is we'll be doing it with an older and slower player. This won't help us.
If we want to get Kroos he should play beside Pogba and Matic, this will the best perfect balance of midfield, like last season with Carrick, Herrera and Pogba, or Juve's trio of Pirlo, Vidal and Pogba.