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.
The competition this season in the champions league will be too fierce for PSG to handle, they do not have that ability to do a back to back win a a row, they can only try and maybe get to semifinals but seeing them lifting the champions league trophy again this season looks like one which I do not expect of them, they cannhope for it in other seasons but not this most probably. Like they always say expect anything in football so I will never say never but to the greater part of my taughts, PSG is not a favourite to win.