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.
I always feel bad about Inter Milan because they can't make it to the final of a big tournament like the Champions League or the semi-finals of other seasons and win the title. I still remember Inter Milan's performance in the Champions League final against Manchester City, where for a while we were forced to think that Inter Milan would win the Champions League title, but that season too, Manchester City won the Champions League title by defeating Inter Milan 1-0. Last season, Inter Milan played the Champions League final against PSG, and we didn't see a good performance from Inter Milan either. When a team is eliminated in the quarter-finals or before, that team doesn't have that much regret, but when a team loses in the Champions League final, that team may always regret winning this trophy.
This is because inter Milan has refused to bring in younger players to strengthen their squad and this has impacted negatively on the and we saw that in the finals of the Euefa champions League finals were they were humiliated by PSG, inter Milan should start a rebuilding stage if they are to come back to competitive football challenging for titles.