The squad needs star players. However, it is not always possible to perform well if there are star players in the squad. In this year's Premier League, we can see Chelsea and Manchester United teams as examples. A lot of talented and star players are in the squads of these two teams. However, their performance is quite poor. Qualification for the Champions League is unlikely to be achieved by these two teams. On the other hand, Aston Villa and Newcastle teams are performing better than Chelsea and United without any star players.
PSG definitely need a score machine like Mbappe to keep their performance stable. Mbappe has the ability to score a lot of goals. But if match strategy and chemistry between players is not good, it is not possible to survive in a competitive platform like Champions League.
If Mbappe leaves the club in summer, which I think will happen no matter what PSG will do, the club will lose so much besides an exceptional striker. I think the whole team will suffer from Mbappe leaving them. They are so much less dangerous without him. Imagine you take him out of the team now, I think they wouldn't have the slightest chance in the Champions League as he is the player who can really decide a game within a blink of an eye. They would still win Ligue 1 as this is nothing special given how much quality they have in the team, but on the highest level Mbappe can't be replaced.