Real Madrid has prepared everything to find a suitable replacement for Kroos. They will never be different even though next season Kroos will no longer play for the team due to retirement. Even Modric, Carvajal, Nacho and all the other players are also waiting for their time to leave in any way, that is the reality.
Real Madrid have never had difficulty finding a creative midfielder like Kroos, but of course they have never been on the same level. This is just a dynamic within a team and they cannot prevent it considering that all players have their time. Kroos' decision to retire may have been the right one for him, but it was probably not what most of his fans expected.
That is one big advantage that Real Madrid has over other teams. With their history and reputation, they exactly know that if they are competing with other clubs for the best talent, most likely they will get the signature from the players. But Kroos was a midfielder on another level. I think his passing accuracy and his consistency was insane. I think there is hardly any player to play the exact same game every single game. It is so important for a team because it gives so much stability. Kroos was predictable and reliable for his teammates.
But sure, at the end of the day another superstar is somewhere in the making. They have great midfielders although I think that Kroos is very unique with his game understanding, his precision and how he understands to exert control over the game.