Every player has their prime, and age also plays a role. Real Madrid is a team that doesn't rely on just one player. Modric and Kroos are no longer young, and their performance has declined significantly. So I never thought Real Madrid would slump just because they lost these two midfielders. In fact, I see the Real Madrid team always ready to regenerate, and now there's Jude, who can be relied on in Real Madrid's midfield.
Despite the fierce rivalry between Real Madrid and Barcelona, even when one team has many problems, it doesn't diminish the intensity of the competition for the La Liga title.
Players have their primes and clubs do have their primes too. From clubs to national teams, they can not maintain their successs and domination for too long time as basically their success and domination come from players and an player generation sooner or later has to hit their physical and ageing limits so that they have to either can not win against ageing and consequent effects result in worse performances over time.
Real Madrid is an example as they dominated C1 decades ago before having big difficulty to win Champions League title in more than 10 years. Suddenly they got another prime with 5 Champions League titles with Ancelotti and Zidane as head coaches and their golden player generation with Ramos, Marcelo, Modric, Casemiro, Kroos, Ronaldo, Bale, Benzema, Carvajal and more.
Now it seems that they are declining again even they have many star players in their club but it's just too early to conclude that. Xabi Alonso and Real Madrid players still have many matches and time to improve and win titles and prove that this prediction is wrong.