PSG had three superstar players on the team and they tried their best to have a better performance in the Champions League but still, they couldn't do that and even the players they had like Messi left the team. I think now after Mbappe leaves, they will have bigger problems but as Enrique said, they will try to fill the place of Mbappe in the team by using all the players they have by having better teamwork.
I believe why they havae struggling to win the champions league is because they haven’t appointed a talented manager, any manager will easily win the Ligue 1 with the quality players available because other clubs are still behind in terms of quality. The champions league as the name implies is the league for the too clubs across Europe so the competition will be more difficult and competitive and if requires a very good manager to train his team to compete.
If they appointed a good manager PSG should be able to win it when they had stars like Messi, Neymar and Mbappe but they didn’t even come close because the coach couldn’t manage them properly.
Maybe with Mbappe gone it would be easy for them but i doubt if that will be the case because no team would say a player that score goals is their problem.