I read that Mbappe's departure will not be free. He gave this statement to the club, so Mbappe must immediately sign a contract extension if he does not leave this season. So it seems that this rumor is only a release clause. PSG still continue to hope he signs a new contract perhaps with a new clause and he will not leave for free. If it is indeed agreed later, it will be a new record. I agree with you, PSG can build a team with new players even though the risk of a decline in the quality of the game can occur.
Source:
https://www.espn.com/soccer/story/_/id/37964018/psg-chief-kylian-mbappe-sign-new-deal-want-stayIndeed. Even if Mbappe leaves PSG now, he won't be for free because he still has 1 year contract. His contract will expire in June 30, 2024. PSG management wants to extend his contract as quick as possible to have a bigger price for his transfer fees. When the remaining duration of his contract is still long, then Real Madrid should pay a higher price. It is just a tactic of PSG management to get a more benefit from Mbappe's transfer.
By the way, Mbappe will break the record on his transfer whether he signs a new contract with PSG or not. Real Madrid must pay more than 200 million euros, it is almost 300 million euros. No player ever reached this price on his transfer fees. Sometimes I think that this amount of money are too much for 1 player.