Mbappe is a great player and one of the best players in the world. Mbappe's chances of winning the Champions League title with PSG are slim. Despite dominating Ligue 1, PSG failed to perform well in the Champions League. This is the main reason why Mbappe left PSG.
I would have been happy if Mbappe had left PSG earlier in the season. He is leaving PSG as a free agent this season. PSG have paid huge sums for Mbappe every season. Paid high wages. But PSG team failed to earn money by selling Mbappe. And that's why many say Mbappe is a bit greedy, and he's not loyal to the club. But I think Mbappe's decision to leave PSG is the right one. And this is the right time to leave PSG.
Yes, if they want the Champions League title for the next few seasons, it will be difficult for PSG to make it happen. PSG must completely overhaul the squad and coach and that will take time. Therefore, Mbappe should be sold, maybe he also wants to chase the Champions League trophy with another team. I also believe that PSG will not develop if Mbappe is still around because the club has given him so much power that other people can't do anything, including the coach.
PSG have had a very strong squad over the past few seasons. They had a lot of talented and experienced players in their squad. But there was no experienced coach. Now PSG has an experienced coach like Luis Enrique, but their squad is relatively weak.
PSG management are likely to give Luis Enrique complete freedom as well as large funds to restructure their squad. Mbappe will not renew contract with PSG as we already know before the start of this season. So Luis Enrique is definitely trying to find a suitable replacement for Mbappe. Hopefully, with the help of Luis Enrique, we will see more improvement in PSG's overall performance next season.