Losing Mbappe would cost PSG really a lot by the way. He is like their everything at the moment. In his absence I don't know how consistent and powerful PSG can be. They have made a good transfers for the future maybe but these new players will need to take their time to adapt and improve their skills to create a solid team.
In case of Mbappe's leaving maybe PSG would look for a solid replacement for him. But for now I don't know who they would go for. Maybe Haaland? Because his release clause is expected to be active by this year so we never know what PSG will do after that.

In the first theory, Mbappe is a great player losing this player can damage the team in PSG as you said and this would be so hard for this team to find a replacement for Mbappe in PSG there is no team they want to lose a player in the level of Mbappe easily and I think PSG can't find alternative players in this team among younger players if they lose Mbappe.
But on the other hand, we can't forget what happened in Tottenham when they let Harry Kane leave the team, this team was not damaged but they even got stronger than before.