Really, wouldn't it be the other way around? This is because several players such as Benzema, Asensio, Busquets and Jordi Alba have not been seen in the EL Classico duels this season. They is the people who play a role in make EL Clasico more competitive and exciting in previous seasons, while there has not been seen a replacement that has the same potential or is close to the players I mention earlier.
I think EL Classico will still be exciting next season even though some of the players you mentioned are no longer in La Liga, because fans of both teams or those who really like EL Classico will definitely still have a lot next season so the excitement might still be seen though it is also true that some of the players you mentioned need to be replaced in La Liga who can at least be a bit equal to them in order to keep EL Classico tense and fierce.
EL Classico will only be exciting when Barcelona and Real Madrid have almost equal player skill. Mbappe's Real Madrid will undoubtedly lead Real Madrid ahead of Barcelona. Barcelona used to have enough money which allowed them to buy players on par with Real Madrid but they don't have that opportunity at the moment as they are suffering from financial problems. If Mbappe is signed by Real Madrid next season, Barcelona will sign a player considering Mbappe's power, speed and skill. EL Classico will be exciting when the two teams are as aggressive as ever with neither team conceding a single point and talking to their opponents eye to eye. Only if they come back will EL Classico be as exciting as ever.