Experienced and talented players like Luka Modrich and Toni Kroos are very difficult to find. Moreover, these two players can change the entire scenario of the match even in the last minute of the match. So I think it is not possible to find a suitable replacement for Modrich or Kross so quickly.
But Ancelotti won't face too many problems without Kross and Modrich. Because Ancelotti has been able to find several talented young players. And if these players can be looked after properly, they can perform better than Kross and Modrich in the future. Bellingham, Camvinga and Tchouameni are all very talented players.
Modric seems to be staying, I do not think that he will be leaving this season, I remember reading that he signed one more season, probably his last season. Kroos however is already gone, he is retired and I do not think that he will make a comeback anytime soon.
Only situation is that we are dealing with a lot of midfielders at Real Madrid, while those two are great, and yes they can't be replaced in a fan way, I disagree that the talent isn't there. They have some amazing midfielders and they are going to keep playing with those young stars, and should be doing fine if you ask me. Maybe it will get worse in some aspects, like mental abilities of Kroos and Modric may not be found at some young player.
However, instead of that, these young players have much better physique and in todays world body matters a lot, a paced quick agile player would make so much difference at midfield, specially at defensive side. This means that I believe they are going to be fine without them, sad situation to see them go, means that I am getting even older, but that doesn't mean that we need to be upset about it, we just need to realize that it is going to be normal and not a big deal at all.