Mbappe has seen his condition at PSG realistically, he only earns money there but has very little chance of winning the UCL trophy. Moving from PSG was the final solution for him who still really wanted to experience the pleasure of touching this prestigious trophy. He is still young, his career still has a long way to go, moving to another club would not only open up the opportunity to win the UCL trophy but could also give him the chance to win the Ballon D'or.
Haha that's made me laugh at PSG's winning a champion league. Yeah, I agree with you that PSG is not a team to win the Champions League because they have a lot of problem in their team. If Kylian Mbappe wants to win a Champions league in his career, he first needs to leave PSG and go to other clubs such as Real Madrid, Manchester united, Barcelona etc. So there is a possibility for him to win something like the Champions league or the Ballon d'Or. The point is that earning money is not everything; winning a champion league or Ballon d'Or is the wish of every player. Besides that you said it right that Kylian Mbappe still has a long time in his career, so he needs to leave PSG first. Kylian Mbappe transfer is always at the news, but no one has ever taken it seriously.