I believe that PSG will maintain their standard as the best club in France. Now that Mbappe is gone, anew player will come out and take over his role. We will see a new player that will be doing all the scoring for PSG. The reason why that player has not being recognized is because Mbappe do outshine him. We have seen this happen in my clubs like Tottenham, when Kane left Son took over. In Real Madrid when Ronaldo left Benzema took over and in Man United when Ronaldo left Wayne Rooney took over the scoring and many more. One thing that might be difficult for PSG is to win UCL.
Under an ideal situation, PSG will really want to maintain such condition such that they will be able to still be the best in the whole of France and still maintain that top position on the table while still having to be seen as the dominating team too on the table, it's still very possible and achievable but then the honest Truth is in the fact that the team isn't in the exact form they have been much before now so there are going to be some changes which all .ay have to adapt to, this will have to be with both the players and the fans, I know it may look like much didn't change but they can deni the fact that a key player which is of great importance left and is definitely affecting them.
A new player will always come and it will be such that they may do much better than the one they are replacing just like the examples you have sighted and it's still possible they may perform leaser in the very start and in the long term thy become better, one thing is sure about all of this teams, they don't go for replacement that will be worse than that which they once had as it will not be progressive. Getting a new exact replacement is something that is and has never been easy but whatever the replacement is, it's believed that with time, such will definitely change to be better and then serve the purpose for which they came in just as seen in all of those teams mentioned.by you.