Real Madrid is a team with a very large player pool. Kroos' departure will not have a shaking effect on the team. I'm sure Ancelotti has many alternatives in mind instead of Kroos. Kroos moved away from football at a somewhat early age. I think he could have played top-level football for 2 more years. Still, it's his choice and we should respect it. Real Madrid needs a quality midfielder for now. Let's see who will do this next year.
Toni Kroos departure will not have much of an impact on Real Madrid performance, Ancelotti still has other players who can replace his role in midfield. It is not too difficult for a team like Real Madrid to find a suitable replacement to replace Kroos, especially now that they have Bellingham who has excellent goal scoring abilities.
Kroos decision was very surprising because he is still able to play at a high level and his abilities are still very stable, but he chose to rest sooner than expected. Real Madrid must prepare an ideal replacement because soon they will be left behind by Modric.
I believe that for now Modric will do more of the midfield work with one of the young players to substitute him and as time goes, Ancelotti will look for a good replacement for Kross. Kross is still sharp and strong but his decision to quit from football is best known to him.
Real Madrid will not miss his departure that much but he will not be forgotten because he has also contributed a lot to the club success by playing his role perfectly. Real Madrid still remains the club that will be the winner or Laliga next season which will be a back to back thing and if the also win UCL it will be 2 back to back in both competitions.