If players like Marcelo and Ramos leave Real Madrid, Mbappe will not be able to make up for the lack of Ramos and Marcelo. Real Madrid will have to suffer for a long time if Ramos and Marcelo are left out. The main reason Real has been playing well without a good striker so far is because their defens and midfields were strong enough. Now Zidane is strengthening the forward lineup, making the midfield and defense worse in return.
I am sure if players like Ramos and Marcelo leaves, Real Madrid will not be going just "hmmm our players are gone, let's put my uncle in there", this is Real Madrid we are talking about, maybe they will not find a player that good, maybe they will find someone even better we do not know, but they are going to find players who are great by world standards for sure.
They had marvelous players 10-15 years ago when Ramos started, we all remember the Los Galacticos period, and then they moved into the benzema ronaldo ramos period after that, well ronaldo is already gone but when marcelo and benzema and ramos type of players are gone, they are going to find another great generation and pay whatever they have to to keep it going. So I do not believe Mbappe will be nothing, he will be the cornerstone of the next generation of greatness for the team.
I partially agree with you. Maybe Zidane will find someone better. But it takes some time. The new player on the team actually needs a lot of time to coordinate with other players. Suddenly new players join the team and do not play well. He has to be prepared. The team has to give everyone a chance to get acquainted. So it would not be a good thing for Real if established players like Ramos and Marcelo left the team.