I think the problem is from the coach mentality because PSG has money and they should be able to buy a professional coach that will be able to use the resources he has to win UCL. Why I said this is because when PSG had the best players in the club that was when their performance in UCL was worst which means he doesn't know how to mange quality players.
Mbappe will definitely be missed by the club but I believe that another striker will take his role and we will be surprised on how he will be scoring goals even though he might not be as sharp as Mbappe because there are some critical matches that Mbappe experience will turn the game around in favor of PSG so those moments will be missed.
Luis Enrique has the ability to be the best coach and maybe it will take time for him to build a stronger team. His arrival at PSG has brought big changes and indeed has not succeeded in the Champions League but compared to the previous coach, Luis Enrique is much better in terms of managing the team and making players more solid. Now they need to find a replacement for Mbappe and if they get a good player then I am sure they will be much better.
Every player who leaves will definitely be missed, especially Mbappe, who is one of the important players in the team, but the arrival of other players will make everything forgotten because the club's journey must be maintained with new stars who will later provide the same thing.