You are right, as long as there is still a contract, everything can be discussed and PSG has more power to be able to keep Mbappe.
If the contract runs out and Mbappe decides to leave, maybe PSG will increase the value of the contract or even let him go, but it looks like PSG will keep him.
Mbappe is an asset for the PSG team and Mbappe has managed to provide many goals and wins for PSG.
It can be recall that Mbappé recently signed a two-year contract extension with PSG but had the option to extend it for another season. However, he has confirmed that he will not exercise that clause, meaning his contract will expire in a year. PSG, facing financial pressure due to Financial Fair Play regulations, has decided to potentially sell Mbappé for €150 million. One reason that is making Mbappe leave is the departures of both Sergio Ramos and Lionel Messi. Both of them had the option to extend their contract but decided mutually to leave Paris Saint-Germain in search of new adventures. This has made Mbappé realize that if these two players, who have won everything and are strong personalities in the world of football, want to leave PSG, then something is not clear.