PSG has only one obstacle that might make things complicated for them. Chelsea is the only club that have defeated PSG in their best form and I believe that if they clash early in the knockout stage, PSG might find it difficult to win Chelsea. However, there are other strong clubs that van stop PSG from defending the title next season.
PSG simply needs to improve their emotional state when they lose early against any opponent. If they immediately show their emotions just because they can't take the lead early, their opponents will easily exploit them, leading to a crushing defeat later in the match. I truly didn't expect PSG to lose so badly against Chelsea in the Club World Cup final, but in the Champions League, I don't think the same thing will happen to PSG again if they no longer let their emotions play out after losing early in the first half.
I believe that they must have learned from their mistakes in that match. Chelsea has also shown other clubs PSG weaknesses and how go defeat them. It's left to Enrique Luis to work on his men so that they will be unpredictable by their opponents next season in the Champions League. Chelsea is not a threat to PSG if they work on their weaknesses, but if they don't, the problem still lies.