Arsenal simply didn't have enough composure and attack ability to threaten PSG. Merino is a fine player but he is not a central forward Asrenal made a big mistake not to buy a new forward when Gabriel Jesus got injured, they where left with only Havertz at that moment and they should have been wise enough to cover a position and buy one good attacker but they didn't and now pay the price.
For the final my candidate is also Inter they look better then PSG.
Apart from the lack of mental calm, Arsenal players are easily influenced by Inter Milan players so they don't play with a high level of confidence. The absence of a pure striker actually doesn't really affect Arsenal's game in that match, but I saw a less than optimal midfield because there were no good passes that could be given so there weren't many opportunities that could be utilized properly.
I favor Inter Milan, but if you look at the performance of PSG players, Inter Milan must be able to match the speed and we know that Inter Milan players have difficulty matching the speed of PSG players so the coach must have options for this problem.