Instead of PSG fan to direct their frustration at their incompetent management that have not been able to buy the right place for their club, they'll chose to attack a certain player. They did it to Neymar few seasons ago and they did it to Messi this season. At this rate they'll soon turn on their golden boy Mbappe.
PSG have a bad coach, a bad midfield and a bad defense but they blame Messi for all their losses. They expect Messi to replicate his world cup form but they forget that the Argentina world cup team played like a team, not that individual football that they play at PSG.
I don't think anybody would ever be replacing Messi in football. I watched Messi from the beginning of his career till now and I can boldly say nobody will ever do it like Messi against.
I think fans psg will regret the behavior they did for messi after messi left for another team and they will see significant changes in the next season. Even for Neymar I would agree more if he went to another team in this season transfer window. He has received less than good treatment from PSG fans and he should have weighed it to decide to accept the offer that came his way.
Mbappe is PSG golden boy and this can be seen from PSG treatment which makes him special compared to other players.
On the other hand I think psg will not be as strong as this season since messi left psg. PSG will find it difficult to compete with other teams in the Champions League and they will even need quite a long time to get the UCL trophy like Manchester City did.