Since Messi has left the team, Mbappe is now the main player in PSG's squad. The team will now depend on Mbappe's performance. PSG management is well aware that now Mbappe leaves the team and their performance will drop badly. They will even lose dominance from Ligue 1. However, PSG has another star player Neymar in their squad. However, due to injuries, he could not stay with the squad for the full season. And so PSG will not risk relying solely on Neymar. Next season we will likely see both Neymar and Mbappe in PSG's attack.
Mbappe was PSG's main player before Messi, during Messi and after Messi's departure he remains the main player. And it seems that your prediction is in danger of being completely wrong: PSG are forced to sell Mbappe, and PSG want to get rid of Neymar. Most likely, both of these players will not be in PSG next season, but I don’t think that even taking this into account, PSG will lose their dominance in Ligue 1.
In several seasons after the 2018 World Cup, Mbappe has proven his playing skills, even when compared to Messi, Mbappe is superior.
As I expected that PSG will release several players in attack but as for Mbappe I still don't believe that PSG will let him go considering he has become a core player there.
By releasing Mbappe's departure, wouldn't it actually be able to bring PSG in difficult times because the player they relied on had left there.
It's very far to compare mbappe and messi because these two players for me have the same performance and you really don't need to compare these two players.
If you really understand that these two players are very much the same, it's just that Messi is a senior player who has more experience than Mbappe, but Mbappe can be Messi's successor as the best player in the future.
On Paris SG letting mbappe go it seems there is no word yet on whether mbappe will leave Paris SG but I think it will happen.