I think the tactic of letting old players go and bringing in a new young quality player into the Real Madrid squad is what has been working for Real Madrid for ages to always be the Laliga and Champions League strong title contestants every given season.
Now, Joselu has let go, for Mbappe to step into the team. It will be like nothing really changed in the frontline of Real Madrid but instead, they made their frontline to be more stronger to go head-to-head with other teams for the title race. By next season, other old players will be let go from Real Madrid to bring in another young quality players. The changes of the old players to bring in young quality players is normal routine for Real Madrid every beginning of new season.
This is a process that we will later call a regeneration process for a team and Real Madrid is doing it well. Indeed, a team has to do something like this, because it is impossible for them to rely on the same players for a very long time, maybe they can do it in 4 to 5 years or at least until the players are old. After that they couldn't rely on him anymore and had to have younger players replace their position.
In my opinion, Real Madrid is one of the teams that has succeeded in carrying out this process very well. They always trust young players and even when senior players are still in their squad. And also young players who are trusted by the coach can show that they can carry out their duties very well so that they continue to get more playing minutes from time to time.
Real Madrid is well aware that even though they are a big team that can bring in experienced players, that is not the only way they can do it, and they can make good use of other options.