The quality of player is what many do think can get a team trophy but this proved to everyone big time that there is more to a team than just having set of good player alone, Inter Milan had their confidence that way because they probably were not expecting PSG to be a problem to them , if you had followed up with how PSG made it to the finals you will agree they struggled so hard at some point, even at the league stage they finished below top 10, so maybe Inter least expected much challenge from them.
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 disagree with both of you. Inter did not underestimate PSG. They knew how good PSG were, and they knew PSG were a better team than them. Everybody knew that. The win against Barcelona didn't fool anybody into underestimating PSG. There was a bit of luck on the side of Inter against Barcelona because it's evident in the fact that Barcelona outplayed them in both legs, but they just conceded so many foolish goals and missed a lot of chances.
PSG were simply way better than Inter Milan. They saw the mistakes Barcelona made and corrected them. They were very good in defence and made sure they finished the chances they got. In fact, the Inter that met PSG was weaker than the one that met Barcelona because at that point, it was as if they were broken. They had lost the league and the cup, and the only chance of a silverware was beating a team that was obviously better than them. They did not lose because they underrated PSG; they lost because PSG was simply better than them.