You have said it all. Inter underestimated PSG due to a lot of reasons. Firstly, they haven't win the trophy before and don't have the finals experience. Secondly, they struggled till they got to the finals and lastly, Inter defeated Barcelona who everyone sees as the best last season. I believe that Inter didn't even prepared properly for that finals. Assuming, they prepared for the finals the way they prepared when they played Barcelona, PSG wouldn't have humiliated them that way.
I hope Inter can learn a lot from last season's defeat, but seeing Inter current condition, which is even worse, makes them even less likely to succeed in the Champions League. Meanwhile PSG is getting stronger, so now the Champions League group stage presents many tough opponents for PSG to face, such as Bayern Munich, Barcelona, Leverkusen, Atalanta, Tottenham, and even Newcastle. All of these matches will be extraordinary challenges as PSG begins its journey.
Inter Milan has not learnt from their failure in the Champions League last season and am even in doubt if they will reach the semifinal stage not to talk of reaching the finals this season because the group stage matches comprise of tough opponents so i don't know how they will manage to scale through. As for PSG, they also have tough opponents but the greatest threats they have is Bayern Munich and Barcelona, the rest opponents aren't that threatening. Newcastle, Atalanta, Bayern Leverkusen and Tottenham wouldn't be problematic to PSG because they are just average teams that were lucky to make it to the Champions League especially Newcastle, i wonder how they will survive in the Champions League games since one of their major strikers namely Alexander Isak has left the team for Liverpool so it's really going to be difficult for them. Besides they are already finding it difficult in the Premier League games, how much more in the Champions League games.