Where players go to different clubs in Europe to build their careers, if they go to PSG, their careers will be destroyed instead of brightening.
It depends on when a player joins PSG, if he joins as a young player and has yet been star player, playing for PSG will help to upgrade his career. At PSG, at least winning titles like Ligue 1 is possible and can win some Ligue 1 titles too. At smaller clubs, even winning Ligue 1 is a dream ans nearly can not achieve it.
Neymar was a very talented player and when he was at Barcelona his brilliant dribbling caught the attention of the entire football world but when he moved to PSG his career started to go downhill and eventually he was forced to leave the club in disgrace. It will be the same with Mbappe, Mbappe is wasting his time from this club, even if the amount of money is relatively small, Mbappe should change this club very quickly. If Mbappe thinks he will make his career brighter at PSG, he is wrong. A lot of people will stop watching Ligue 1 if Mbappe leaves PSG.
Neymar and Mbappe joined PSG with different positions. When Neymar joined them, he was already recognized as of best players who can challenge Messi and Ronaldo. He together with Bale and Hazard were promising players who were considered to win Ballon d'OR.
Mbappe joined PSG as a young player and PSG actually helped Mbapped to become a better player. But in last three seasons, he has been no longer happy in PSG because he has become a super star player and want more than just Ligue 1 title.