the situation will be difficult for PSG if Mbappe leaves them. especially if PSG is not ready to replace him in the team. they will negotiate for Mbappe to stay with PSG. but Mbappe wants development and challenges. and the Premier League is a good place for him to develop alongside tougher competition. we have to realize that the French League is not as tight as the English League. maybe Mbappe realized that.
Mbappe really wants to leave PSG on the grounds of growth and further development in his career as this has always been the bases and reason for wanting to leave PSG because the French league in it's entirety isn't helping with development and it's aswell looking more like a mediocre league as the day goes by.
Premier league isn't a place for development either, if mabppe moves to England he will only be in competition and not develop, england isn't a place for development but advancements due to the nature of the league, it's competitiveness and the level of attention it attracts aswell ares such that only players who are looking forward to advance on bigger platforms gets the best out of premier league but for developing players they may not Excel that much as they will in other leagues ,I think mbappe will do even better if he goes to Spain rather than england.
I don't think it will be a problem for PSG if Mbappe leaves in Ligue 1. But in the UEFA Champions League, They will face big problems. They have a match with Real Sociedad in the UEFA Champions League and I think without Mbappe they will struggle against them. PSG has achieved a good lead and I believe that would make them comfortable in Ligue 1. If they had not had that, they probably would have been in danger.
But to be honest I don't see Mbappe leaving. Because if he were to leave, we already would have heard some rumors by now. Real Madrid also said they wouldn't be buying any new players, so Mbappe wouldn't be joining Real Madrid. And I don't think any other team can afford him.
If Mbappe leaves PSG it will affect them both in the uefa champions league and the domestic league, because Mbappe is PSG's point man, and PSG does not have any other player who can fill Mbappe position if he leaves for another club. Real Madrid should look for another good striker to sign because PSG will not be willing to release Mbappe.