It is very likely that Mbappe will remain for this season but Messi will surely leave, because I believe he has played his last match in the club. If Mbappe stays and they reinforce the team with good players that are not super star, the team will be balanced but congesting one team with 3 super stars.
If Neymar leaves, Messi leaves and 5 new and good players are added to the team for reinforcement, I believe PsG will have good results for the next season champions league.
So far the quality of Ligue 1 has not been able to compete with other leagues to reach the top of the UCL champions. It is evident, the power of money is still quite difficult to get a champion instantly. Neymar, Messi and Mbaape they are good but don't have hard work. It seems that they are too comfortable with a large salary. It seems that they have to change the mentality of the players to work hard. As for the League, they have had many titles. They just need to improve the quality for UCL.