There is some good news for PSG. Neymar is already coming back to the team after the elimination of Brazil.
Messi and Mbappe are still going on in the tournament but they are much less injury-prone than Neymar obviously. Neymar was really demoralized after being eliminated by Croatia but there is still a very long time ahead for PSG's next matches.
He can use this time to gather himself up and focus on his game at PSG again. PSG need these three important players not only healthy but also mentally strong. The Ligue 1 is pretty easy for them maybe but the Champions League doesn't forgive any weakness.
The most vulnerable is Messi, if he and his Argentina make it to the final round. if not injured, the effects of fatigue can greatly affect him to face his matches with PSG after all leagues are rolled out again. because after all, this world cup tournament will really drain the energy and mentality of the players who take part in this championship. for Neymar, I'm not sure that he has fully recovered. but he has enough rest time, before returning to play with PSG.
Mbappe, he's still young I'm sure he'll be fine after the world cup is over and come back to the club in top shape. on the other hand, the elimination of the Portugal squad over Marocco. several PSG players who defended Portugal will return to the team, such as, Vitinha, Nuno Mendes. Unfortunately, Danilo Pereira is reportedly injured. there is no certainty as to how long it will take him to recover. another formidable defender who will still be in Qatar is Hakimi.
so there are 3 PSG players who are still defending their national team in this world cup. even so, I hope that all PSG players who defend their national team will not experience problems after returning to PSG.
I saw a lot of people are talking about how teams of stars of PSG, everyone knows them but Messi, Mbappe, Neymar, were favoured by the referees in the Qatar World Cup. I don't agree with this but there is too much speculation. Therefor, it can be considered expected that the teams of Neymar, Mbappe and Messi came this far and Mbappe and Messi will continue. Things are looking good for Qatar, PSG and their stars. Personally, I don't want to believe in the speculations or accusations of the briberies or gossips. I just want to watch good and enjoyable football. And stars being shine and make our time watching football enjoyable. But, it is very hard to not listen what everyone is talking about. Nevertheless, I also agree with your point, since Messi and Neymar are considerably older, they will be more affected by the fatigue from this World Cup. Even the both are quite professional, the psychological aspect of a big and important tournament will take its toll from these players. But as you said, Mbappe is quite young and he will be less affected. Still, things are looking great for PSG after this prestigious world cup for them.