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.
I think so too, especially now that PSG is exaggerating Mbappe's presence so much that other players are covered up even though basically a team's performance depends on the performance of all players in all lines, not just one player.
As an individual, Mbappe is quite great and I think for now he is one of the best players in the world, but for PSG I believe they would be better off losing Mbappe so they can build this team with a strong foundation and also all lines with the same quality of players so that it can lift team performance as happened to the Tottenham team.