Since Messi has left the team, Mbappe is now the main player in PSG's squad.
Mbappe is always a main player in PSG since his transfer from Monaco. He joined them as a young star player but now his position improved a lot as one of biggest super star players.
The team will now depend on Mbappe's performance
He had great contributions for PSG in the past seasons and PSG depended on Mbappe performance a lot.
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.
Neymar might be sold to another club too as Mbappe does not like Neymar and a best friend of Neymar, Messi will leave PSG to join Inter Miami.
However Mbappe is not a player who can help PSG to win all matches and win titles. He needs to have support from good team mates. A club can not win Champions League because they have Mbappe in the team. They need many good players in defensive and midfield areas as well as a good coach who can maximize player ability and is able to use appropriate tactics to beat strong opponents.