PSG already has many star players with expensive transfer fees and also with expensive salaries. If they can buy the title, then they will. PSG must maximize all the potential of its star players to dominate Europe by bringing as many titles as possible to Paris. It doesn't take a lot of other star players if you can't get the title and it's just a waste of money without a prestigious title.

As for Mbappe, they could sell him as they may no longer be interested in staying any longer. PSG already have Messi, Neymar and several other players who can help them win the title. Mbappe will be sold and they can save quite a bit of salary and replace some of the aging veteran players with some relatively younger players.