PSG performance in Champions League decreased after the appearance of Messi in Paris, in PSG squad. It can be a coincidence but can be one of actual reasons too. I don't think it is coincidence and Messi is one of their problems. One more star players, less contributions from attacking players to their proactive defense. They have become more vulnerable to counter attacks and in Champions League, it is very deadly weakness.
I also think so, Messi has not made a major contribution to PSG in the UCL, Messi has been at PSG for 2 seasons but there has been no significant PSG performance on the European stage, my assumption is because I see Messi as the best player in the world who should be able to make a difference with the other players on the team but this is not the case.
I think Messi's departure from PSG will not have any effect even if PSG can find a replacement in the same position better yet it is possible for this team to be much better.
Indeed, Messi is one of the best players in the world, but at the same time when he came to Paris Saint Germain he was no longer young, you could say his glory days had declined. The problem is that most people expect a lot from his arrival, while Paris Saint Germain is his second team, he did well at Barcelona but we don't know his abilities when he left Barcelona.