Real Madrid has a very solid center of the field with Casemiro, Modric and Cross. As long as these guys are doing their job at Real Madrid everything is fine. I don't belittle the merit of the attack in the person of Vinicius and Benzema just a lot more depends on the midfield than he does on them. In the championship in terms of debt it isn't noticeable that not everything is so smooth with the defense of the team.
Seville is great for showing good football, it's just that initially and even now the team is not as ambitious as Real.
Casemiro is 29, Modric is 36 and Kroos is 31 years old. I am not saying that they need to get younger players, keep Kroos and Casemiro a few more years if you want, it is obvious that Modric needs to be replaced at the very least, but that is still just a few more seasons of good midfield for them.
I get that they may look at other positions for a year or two, get great defenders and great attackers first but eventually they will need to replace these players. They are all great, they are definitely a great midfield, nobody is saying they are not good anymore or anything like that. I am just saying that they are not as young as they used to be and their time is coming in max of 3 seasons at the very most.
Although midfielder or Real Madrid have over age like Modric have been 36 years old and Toni Kroos 31 years old have best performance right now and they keep consistent on every match. Looking they can playing well on champion league and La Liga give something for fans not worry about age just number only. The same with Benzema have been 34 years old but keep consistent to make goal and assist so far and become top scorer in La Liga with 15 goals.