After making sure Mbappe leaves, PSG is obliged to find a replacement. The coach must start making the players independent of Mbappe from now on. In addition, they also have to buy several new players to patch Mbappe's departure. The latest options I read PSG want Osimhen, Joshua Kimmich, and Leny Yoro. They want to make a big change with those players. But it is not easy, because many other clubs are also eyeing the services of the three players.
PSG didn't make sure Mbappe left the club, as a matter of fact they are disappointed that he is leaving the the club for free after they have invested so much money in him. They were actually warned to sell the Frenchman when they had the chance and they could have made a huge amount of money from the deal, but they hesitated and convinced him to sign a new contract, but all that has backfired right now.
Having said that, i do not know if PSG are actually looking for big name replacements for Mbappe, the club had said at the beginning of the season that they were going to change their pattern of recruitment, from signing big name players to instead signing mostly homegrown talents and building a team with them, we'd see how well they stick to this and what they do in the summer transfer window.