The best scenario for Luis Enrique and PSG is to release Mbappe so that the focus of the other players in the squad is not divided by the contents of the transfer. There are many things that Enrique has to fix, not just Mbappe alone, my prediction is that if Mbappe still refuses to extend his contact then he will be released next season.
PSG need to get money from Mbappe so they will not release the player. They will not terminate the contract and let Mbappe goes away as a free agent. They can reduce price at the end to sell Mbappe but they can hold the negotiation to bargain as highest transfer price as possible.
If Neymar stays at PSG so he can be relied upon for another two or three seasons, Enrique only needs to organize the squad from the defensive and attacking midfielder sectors.
Problem with Neymar is he is very vulnerable to injury and PSG can no rely on Neymar as latest seasons show that Neymar usually was in hospital by injuries when PSG need him most.
If you read the news about their transfer market, the arrival of several new players made Enrique have to let go of several players to streamline the squad, I think it is very likely that players returning after a loan spell like Icardi will be released in this summer's transfer window.
Yeah he has to sale off many players he does not plan to use in a new future to save salary budget for the club and to have vacant positions for new players. Definitely he does not want to lose Mbappe but the future of Mbappe is not controlled by Luis Enrique.