Modrich and Kross were two of the most important players in Madrid's squad. We have seen many times that the brilliant performances of these two players changed the whole scenario of the match. And Madrid won important matches. Even in the last season we saw excellent performances from Toni Kross. If these two players leave Madrid at the same time, the bad effect will definitely be seen in the Madrid squad. But Ancelotti added Bellingham to his squad as a replacement for Modrich. Bellingham has already proven his ability. He is now an excellent attacking midfielder.
Madrid has an experienced coach like Ancelotti. So it is expected that Madrid's performance will be stable in the next season as well. Madrid's squad is also quite strong. Madrid will also try to win the La Liga and Champions League titles next season.
While age had some effect on Modrich's performance, I didn't notice much of an age effect on Toni Kroos's performance. Toni Kroos has performed well in Real Madrid's jersey in the last season and he has also shown good flashes for his team, so if Real Madrid were to extend the contract with this midfielder for another season, this player might not have disappointed Real Madrid at all. As Real Madrid have decided against them, there is nothing they can do next season without these two experienced players. However, since Real Madrid is a professional team, we expect their performance to be stable. The two experienced players may have left their club but there are still plenty of young players in Real Madrid who will certainly do well in their midfield and will not miss the absence of these two stars too much.
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.