With Messi leaving, PSG will lose power, we can all guess. He hasn't had a bad performance this year. The fact that he will leave the team has undoubtedly made it necessary to reinforce the team. On the other hand, if Mbappe leaves the team, it will be a very weak PSG offensively. They may have to spend serious money to fill this gap. I'm very curious about next year's PSG offensive line.
What's the point of spending a lot of money when you get the same result every year. That simply mean they are doing it the wrong way.
PSG needs good talented players that are consistent in their performance, not too much injured like Neymar, not old as Messi, with all respect to what he did with the team this year.
PSG will spend for new players for sure this summer, hopefully they can add something to the team.
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.