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.
Assuming is Arsenal that meet PSG in that final stage, I don't think with would have ended up in that 4 goals, because they know how to play them to take advantage of their defenders to score goals to lift the trophy but PSG took the advantage of inter weak defenders to secured the trophy from the UEFA champion league last season.
Even though inter Milan have not win UEFA champion league trophy but they met club that was stronger than them, which is what happened to inter Milan not to stop PSG in that final game despite the kind players they used. Last season is gone and many cluds have learned lesson from the last season tournament that will guide some cluds to surprise people in this season through the help of their new players.