Furthermore, speaking of Real Madrid, you're right. Real Madrid can't continue to rely on their aging defenders. Their performance is certain to decline and they are also prone to injury. Therefore, Real Madrid must bring in new defenders to strengthen their defense and improve. However, after I investigated, Real Madrid has actually brought in three new defenders: Dean Huijsen, Alvaro Carreras, and Alexander Arnold. All three players are quite formidable defenders. So Real Madrid should be able to strengthen their defense even more with these three new players.
Huijsen is a promising central defender while I don't know of Carreras and Trent does not have good defensive skills. Real Madrid surely knew defensive weakness of Trent and they perhaps plan to use Trent mainly for attacking support. Real Madrid will not accept to lose La Liga title to Barcelona again and they are improving their squad with new players.
Barcelona have a young team that can grow up more but their brightest star player Lamine Yamal shown some signals of being distracted by parties and unhealthy living styles. This can harm Barcelona attacking power in next seasons and give Real Madrid kind of good opportunity to not only challenge Barcelona but also take trophy back.