Real Madrid manager Carlo Ancelotti has been rotating his midfield players since the start of the season and that's because of the fact that his options in the midfield positions are all world class players players. In a team that has the very young, energetic and well performing midfielders like Aurelien Tchouameni, Camavinga and Federico Valverde, it'll only take a very good veteran midfield players like Luka Modric and Toni Kroos to still be able to feature in the team. Modric's career is nearing the end even when his performance isn't declining because I don't think he'll still play professional football in three seasons from now. It's believed that this will be his final season with Real Madrid and the Saudi League will be his next destination.
Real Madrid have a much younger squad this season and they do have players who have above average quality for players their age. Mbappe's presence can of course add depth to the Real Madrid squad because his typical players work hard and have good skills. Mbappe was predicted to be Ronaldo's successor, but there was too much drama surrounding the player, even in the previous transfer market he disappointed Real Madrid fans with his decision to stay at PSG because of the offer of a salary increase to him.
Real Madrid is lacking in the defender position and I agree that they have to look for some young talent and finding a strong defender is much more difficult than looking for a forward. Currently, Real will have many important matches in La Liga and the Champions League and they need several additional players in the upcoming transfer market.