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It is because of the fact that Real Madrid management knows very well that they need the experience of the their veteran midfielders Luka Modric and Toni Kroos that the club made sure they didn't leave during the summer transfer window. But that doesn't mean that they're gonna remain at the club till whenever they'll call it a day in their illustrious career. The young midfielders in the team has the whole of this season to learn from their experience as the two legends of the club might leave at the end of the season.
Luka Modric and Toni Kroos are unlikely to get a contract extension from Real Madrid after their current contracts expire, this could be the last season for both of them playing as Real Madrid players. Kroos can still perform great throughout the season even though he will be 34 years old next January, meaning Kroos is still younger than Modric, who turned 38 last month. Kroos has the possibility to stay at Real Madrid, but that may not be the case with Modric.
Moreover, Real Madrid probably won't get much influence if the two players don't get contract extensions at the end of the season, meaning that Real Madrid still has great midfielders like Federico Valverde, Aurélien Tchouaméni, Eduardo Camavinga and several others. They are still young and can still improve, so Real Madrid do not lack anything other than composure from senior players.