Mbappe leaving is for the best interest of PSG. They have been winning the domestic league without and with Mbappe. When Mbappe and the other stars were they couldn't win the Champions league, which is one only trophy that they are lacking currently. Even if Mbappe had remained there for the next season they will still not win the Champions league and they will keep paying him the big wages he has been earning there. Now with Mbappe gone, their wage bill has been reduced and they are still going to win the French league 1. So for me PSG are the one that's going to benefit on the long run. They have opportunity now to bring players that are going to be loyal to the club. It was obvious that Mbappe wasn't loyal to the club any more.
If seen from one side, this is true, but it cannot be denied that Mbappe's contribution to the PSG team is very large, Mbappe is one of the best players in the world so it is not easy for PSG to find a suitable replacement.
However Mbappe departure does not need to be deeply regretted, PSG is a team with strong finances, they can rebuild this team to be able to meet its targets.
I see that PSg management is too ambitious to win the CL title so they spend a lot of money, and it has proven to be a failure.
PSG management should be able to reflect on the success of Manchester City in building a team through a long process.
Now, as usual, PSG has started to aggressively recruit several players that have been rumored, and I think it will still be quite easy for them to defend the Ligue 1 title even without Mbappe.