I think it's legitimate if he leaves Paris Saint Germain without thinking he owes the Parisian club. Because everything in the contract is something offered by Paris Saint Germain, and of course as a player who is offered all that he has the right to take it. I prefer to say he still appreciates Paris Saint Germain and tries to finish his contract first before he leaves. But would Paris Saint Germain let him leave without taking advantage?
Players like Mbappe will think he does not owe anything from PSG. He possibly thinks that his career can be the same if he moved to another club, not PSG, after leaving AS Monaco. He could be right and the contract with PSG does not have any term like if he could not help PSG to win Champions League, he will receive less salary so it's fair enough for him to leave when the contract ends.
I understand that it's fair enough from PSG side to think Mbappe owned them a lot but between a club and a player, there is only contract in between. If a contract expires and can not be renewed, a player will be a free agent and free to choose any club.
Yes there will always be a limit between the club and the player, which means there is no contract for him to keep playing his entire career at the club. For Paris Saint Germain I think it would be better if they sell this player in the near future, at least they can get the amount of money than they have to release without getting a penny. Although I understand that they will be a rich club that might not care about that, but they also have to remember that business in football cannot be separated, so it's better to do business at the last time. If they postpone it any longer, then other clubs will plan for when the player's contract expires.