No, I don't think they'll be able to repeat that feat. This can happen to a very strong team, but teams that have won have rarely repeated the feat. PSG is a great team and has always shown it, but it must be said that by letting Donnarumma go, they've weakened themselves considerably.
Since the competition was renamed to Champions League, the only club that has ever won the Champions League trophy back-to-back was Real Madrid, so I totally agree with you that PSG can't retain the Champions League trophy this season. No doubt Donnarumma is a good goalkeeper, but I think Donnarumma absence in the PSG team won't have a negative impact on PSG's performance in the Champions League because PSG's new goalkeeper is also good at what he does.
Real Madrid has been the team to retain the Champions League back to back no doubt but this doesn't mean that other teams will not have the same opportunity to do what they did if they perform well like Real Madrid did he they won the champions League. PSG has shown sign of improvement in their performance and they have quality players to back their performance up I see nothing stopping them from winning the competition for the second time in a roll unless they get injuries or meet an opponent that stops them on the way.