Last season, PSG were not strong at all. They could not even take the first place in the group losing it to Benfica! Of the 3 stars only Mbappe was really useful (Neymar at the beginning of the season, but when it came time for the decisive games, he was already blown away). If you look at the “brand”, then they lost to the “great” Bavaria, but in fact we all saw that Bayern was frankly weak last season. In fact, a weak PSG lost to a weak Bavaria. It was the end of their cycle, now we will see what the new team will be capable of.
I think Messi also did ok in the Champions League as he had 8 scorer points consisting of 4 goals and 4 assists and Mbappe had 10 (7+3). But the offense hasn't really been a problem for PSG in the Champions League even when Neymar missed important games. PSG could never be consistent for 90 minutes and even worse than that it was almost guaranteed that they are making one big mistake at some point in the game and whenever that happened, they fell apart despite leading the game still. They need a compact team that knows how to defend a lead during the game and of course a two goal lead even when a mistake happens. Neymar is a player who starts complaining immediately as soon as the team concedes a goal and he never puts work into the defense, yet complains. These emotional collapses cost them the games often times.
In 180 minutes with Bayern, PSG scored zero goals (and we remember that Bayern were unusually weak), useful statistical actions in the group do not cancel the fact that PSG lost the group and fell below Benfica. As for the need to be able to defend the advantage in score, just when you have Neymar and Messi in the team, then you have no chance. Maybe the team can tolerate having one not very active player, but two is already too much (although at the top level even one is too much). An effective defense probably requires more strength than even an attack where you choose the direction and pace yourself, and in defense you try to block all zones and predict the opponent's actions. Therefore, the weaknesses of PSG were quite natural and were not the consequences of emotions or simply "unluck.
Emotions definitely played a role in some of their losses because they gave away leads in totally stupid ways and I remember a two goal lead in the Champions League and when they conceded the first they behaved as if they had just lost the game despite still being in the lead. They have never been stable emotionally either.
But I agree with you with everything you said about offensive players not contributing to anything else other than the offensive game, which you described as not very active, but that is the problem. They can do miracles with the ball in the opponents half, but they are missing when the opponent actually has the ball and yes, a team can handle one such player, but PSG actually had three of them who were not willing to work defensively. Neither Neymar, nor Messi, nor Mbappe were known for working defensively.