Compare the ages of Modric and Cross, and you can imagine how many more seasons Cross can play in Real Madrid if he wants. Last season, Real Madrid did not have Bellingham, and in the new transfer window, Real Madrid can sign a contract with another talented player, this is not a problem when the club has money. Modric is technically a very talented player, this is what allows him to play at such a high level despite his age, but he will not be able to play at this level for a long time.
The talent and experience they both have in football history and in the league is why Real Madrid did not sell them, because if they had left the club, Madrid would not have been in the form they are now because they need players of that caliber to coach and guide the team to success.
But, in the eyes of Real Madrid fans, Jude Bellingham has been a successful addition due of his performance; if Madrid signs more players of his caliber in the middle, they can do without Kronos and Modric.
In fact, Kroos and Modric contracts will also expire at the end of this season and yes, it is unlikely that Real Madrid will extend their contracts either because Kroos and Modric are likely to enter and/or choose to retire. Although maybe they will still play, but it is likely that they will play in a league that does not have the tough competition like Real Madrid and other big European leagues have.
But yes, it seems that Real Madrid has also prepared a team with talented young players for next season when Real Madrid is without Kroos and Modric again. Because after all, Real Madrid already have midfielders who also have great potential and they are Tchouameni, Bellingham, Valverde and Camavinga. So yes, the Real Madrid squad will still be good and great even if Real Madrid no longer has Kroos and Modric in the squad next season.