Well its good for PSG that he is staying,a lot of players are leaving PSG and if he leaves they would be really bad next season. They still need to sign a lot of good players if they want to be an elite team like before and with Mbappe they now have hopes and can build a team around him for next season.
As others in this thread have already mentioned a few times, it is actually not good and definitely not in the interest of the officials of Paris Saint Germain if Mbappe wants to stay another year to fulfill his contract but at the same time does not want to prolong it. This means that the most valuable football player in the world will leave the Club for free and it is totally understandable, that PSG wants to avoid that at all costs. They would rather sell him now.