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One of my favourite scenes in the show. Everything has been leading thematically to this moment. It's genius writing, Reigen is temporarily given great power but he doesn't use it to flex, he stops everyone by telling them how they're behaving like children, not by fighting. https://twitter.com/MobPsychoOne/status/1248406784995233795
Mob temporarily uses his powers to reflect the themes of the entire show, that true development comes not from powers or quirks but from genuine character growth. He unintentionally gives Reigen, the most mature person in the room, his power.
But instead of using powers to act like some kind of crazy super saiyan, he thinks his foes are simply using tricks. To him, they're grown men playing with toy swords and special effects, trying to beat up kids and achieve "world domination"
This is completely antithetical to Mob's character arc of true growth through connections and personal development. Reigen schools them, making them question how they've lived their lives.
In fact later on in the series, everyone except the gravity guy ends up getting a job and becoming normal everyday people, trying to live up to the wisdom Reigen gave them and becoming useful members of society.
At the end of the day what won the fight wasn't overwhelming power by Mob cutting loose (something every other anime ever would advocate), it was Mob choosing to trust in the words of somebody who told him something logical, you don't need to fight. Leave things to grown ups.
Another important moment of symbolism. While the Gravity Boss is giving his monologue about how special he is because of his powers, ONE chooses to start portraying him as a baby, showing his mental maturity despite his old age.
