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.
We can't blame Messi. I saw Messi's performance at PSG. I wouldn't say he played badly. PSG's poor performance in the Champions League was due to their poor match strategy. The bonding between the players was not good. PSG's overall gameplay was weak due to the weak link between the midfield and forward players. And PSG's coach is definitely responsible for this. PSG's gameplay has been completely erratic this season. If the match strategy is not good then good performance cannot be expected from the team. If PSG want to do well in the Champions League, they definitely need an experienced coach. If the coach is not experienced, in adding all the best players in the world to the squad, PSG management will not get the expected performance from the team.