If Messi and Neymar both leave PSG at the end of the season I think this would leave a very negative effect on PSG's overall performance. I know that PSG haven't been much impressive even with this trio this season however the reason is Galtier's strategies mostly as it's seen clearly. The right manager for the team would be a big step forward for the team about making their dreams come true especially about the Champions League title.
However in case of seeing these two very important players leave the team even the right manager would have to deal with some serious problems about achieving their biggest goals. I wonder who would make a new trio with Mbappe in the attacking line.
Paris Saint German suffered a tremendous season, although they'll be crowned champions of Ligue one which have been a regular trophy for the club. Parisians have to make a bold step inother to matchup strength with top clubs in other competition. Messi and Neymar exiting the club, will triggered inconsistency in league games for the team because these players are very important for the squad. Christophe Galtier tried his best to take the club higher but he failed as the previous Managers. Thomas Tuchel couldn't give them the UCL, Mauricio Pochettino didn't meetup and now is Christophe Galtier who's probably struggling in the managerial activities for the club.
What a horrible season for Paris Saint-German, they lost the champions league easily against Bayern Munich, in the French league they had unstable results and they lose many points while they could have had a better performance because of the players they got, also Paris Saint-German will lose many superstar players like Messi and Neymar at the end of this season which can be hard for them to find the players to replace with them.