Luis Enrique has taken on many such challenges before and knows what to do when a key player leaves the team. Mbappe is certainly a good player and has contributed enough for the club so there is no reason to think that the club's performance will stop if he leaves. Since they have good financial support, they can definitely buy quality players for this money. In a few games of the season that just ended, we saw Luis Enrique deliberately benching Mbappe and giving all the other players a chance to try and figure out how their team would perform without Mbappe. The performance of the team has not been very random so if he adds some new players to the club then the performance of the club will be better than before.
Mbapppe wasn't just any key player that left the team such that you can just get to say Enrique is used to key players leaving and he will know what exactly to do when they leave, with mbappe it's really going to be different because usually mbappe acts as a backup for him till he's able to get that perfect replacement for whosoever that key player who left the team is, so now replacing the replacement hits very differently.
Getting used to this is really going to be difficult, even when he tried reducing his play time because he was trying to help his team mate to getting used to playing without him, they couldn't just cope completely and that was why in important games such as the champions league games, he was still bringing in up to play such vital and important games for them so it's actually going to be a bit hard than it used to be, except they get a perfect replacement almost immediately which I don't see really getting to happen because finding such a player is difficult even if they can afford it.
PSG may spend the whole season getting to be able to get the perfect replacement for mbappe because he's got the team placing a good level of dependence on him in a very long while which isn't something they can just drop at once. I know this is a chance for many good players to get to be able to showing how much they can be of importance to the team in that they are now having an opportunity that needs to be utilized due to the absence of mbappe.
PSG may be late realizing that after doing so much, there is no point in keeping a player who doesn't want to stay at their club or wants to leave the club every season. That's why this season when Mbappe's contract with Real Madrid was discussed, PSG did not take it too seriously and they did not put any obstacles in Mbappe's contract. With the amount of money that PSG paid Mbappe every season, they could buy a number of quality players who would be loyal to the club and who would be interested in giving the club a lot.
Luis Enrique may not have felt the need to buy any additional players because Mbappe was there last season, but since he will be leaving this season, Luis Enrique may be able to sign multiple players and have a long-term plan with them.