I don't think they'll find a full time replacement for him in the end. To replace such a player you have to completely change the squad, which PSG categorically do not want to do. They do not want to change their transfer policy and this is their main problem, they think you can buy a player for a lot of money and only from that he will play better, but no.
With time running out in this season's transfer saga it looks like PSG will be starting with something new and I think that's well thought out by Enrique.
The plan to bring in Osimhen and Kvaratskhelia is also likely to fail this time due to the difficulty of negotiating with Napoli about the large release clause but indeed this could be a re-start for PSG so that they know what to do when there is no Mbappe in their squad as well as to see which attacking gaps are weak when they do not have a player of Mbappe's caliber who always made hopes for PSG last season.
Until now PSG has only brought 3 players whose positions are only in defense and midfield and this is clearly a little lame with the loss of Mbappe in attack. But even so PSG can also still take advantage of existing players and slightly change the scheme where Mbappe's position can be filled with several young players like Barcola or change the formation where 3 strikers who are always a mainstay are changed to 2 strikers because they also still have good strikers like Ramos and Asensio.
I don't deny the fact that PSG may change dramatically after Mbappe leaves, the only question is in which direction, because it's quite possible that they may not develop adequate tactics for the new season without Mbappe and then their results will be lame at least at the beginning of the season. Although they have no competitors in Ligue 1 you could say