Barcelona's defense is not enough strong. PSG can take advantage of this opportunity. If Mbappe and di Maria play well, they will be able to knock down the Barcelona defense. But if Neymar is in the team, the players are more cheerful and strong mentally. I'm not sure if Mbappe will be able to perform well without Neymar.
The group stage is always unpredictable! But I agree with you that PSG can easily beat Barcelona in the upcoming match... It's not the first time I notice that Barcelona have to win points in return matches...
I am not sure about the "easily" part for the first game. Barcelona have a big advantage because of playing in Camp Nou. They have also not been having a bad period lately excluding the loss against Sevilla in CDR. It is also said that Neymar won't be able to play in the first game. I think PSG don't have an easy job in the first game.