PSG have made a hugh mistakes in the past and it's currently affecting them till the present. Les Parisiens sacked most of the valuable coaches they had and filled the team with big names that are proudly reluctant in games. A team with so many stars, how do we expects an impressive performances from the team? Ofcourse there performances will always decline. The problem of PSG is very simple, Let Nasser Al-Khelaifi allow Kylian Mbappe push the exits door at the end of the summer, the time use in exploring options for the Frenchman to stay, that same time could be spend on valuable projects. Time for Les Parisiens to move on and push for major trophies and significant talents.
Having quality players but not being supported by a good quality coach also doesn't have any effect, this happened to PSG in previous seasons. PSG will have to give Luis Enrique a chance to make changes a few seasons in and there is no way he can make a difference in a short time to the expected results. This season let Luis Enrique only win the Ligue 1 title and next season he will be given another target if the players he wants can be brought in.
Just let Kylian Mbappe go if that's his decision. Management's job is to look for players who are responsible for the progress of the club, not just hoping for a big salary. If a player no longer feels at home at the club, don't expect him to be able to play at his best level and it's best to let him go and look for another replacement so the coach can build the ideal squad he wants.