For me, PSG's dominance will end when Mbappé leaves, I know that they feel safe with the player, and in fact they do, but basically when we can think about what they can do and achieve, well for me it is a fact that the only one who moves that team is the french, and he doesn't understand it, due to the whims of the player in the team he already has everything, he is the only leader, he already ran to messi and neymar, that was not what he wanted to, ? According to Ney, the player was too egocentric and he only had to do things the way he wanted them, and if they were not done that way he would basically make them leave the club, so this player cannot be given much power, if he goes to Madrid I am sure that he will not succeed with his whims, because the players there will not do what he likes, things are different there.
You are you right but with how wealthy PSG is the team management will surely look for a replacement before then, although Mbappe performance on PSG is something they can never forget easily because Mbappe is one of the reasons for most of there achievement so his absence will easily be noticed but however one thing they most no is that there is every likelihood that a player will always like to try there career on other team, so it is expected for PSG to find someone that can handle PSG the way Mbappe did so that there reputation and performance will be intact, although just like I mentioned on most of my post PSG will mostly find it very difficult to perform if Mbappe is not longer with them but with time they will learn to adjust, however there are even more good quality players on PSG that could easily perform well so perhaps irrespective of the challenges they may have in terms of performance but the team still has a chance to maintain there performance a bit.