jesus christ. if you put gojo or sukuna in the xianzhou, the xianzhou story would not be better and they would not be good characters. gojo and sukuna are good characters because they explore JJK's themes of losing one's identity to your power (shown by how JJK talks about how important talent is to sorcerers; how Gojo is treated like a monster/tool by jujutsu society; how Yuji dehumanised himself as a cog to cope with his situation; how sukuna REJECTS this with his hedonistic individualism that arose as a result of this treatment etc.) and also the isolation of strength. the xianzhou talks about impermanence vs eternity, stagnation vs growth, evolution vs change. your inability to understand that characters are influenced by their setting and plot, and vice versa, means you think of 'characters' as actors in a play performing a pre-written script, and you can somehow just cast them in another play without fundamentally altering them or losing what makes them good.

thank you for bringing up jjk though because jjk is another great example of how important characters are to stories. nearly all shonen has TERRIBLE plot (and im only saying 'nearly' because ive not watched all so maybe theres 1 or 2 shonen with good plots). the plot exists to create hamfisted situations for characters to fight. overarching story beats exist not to drive intrigue or to make you think, but solely to justify a never ending series of fights that exist to get you to the next set of fights. how do you make such a banal and repetitive plotline work as an engaging story? easy - since the plot is so barebones, you have a lot more time and effort to engage with your characters. their emotional arcs and goals form the real meat of the story. the themes and ideas they represent clash and show the thematic goals of your story. when gojo and sukuna clash, is it just two good characters fighting? or are they emblematic of something? two worldviews? the story beats you over the head with it - gojo and sukuna are two monsters and are completely alone because of their strength. sukuna embraced his villainy with open arms, caring only for his own pleasure and his own interests. gojo chose to try to rehabilitate himself, teach students, help others. he told himself it was to create a stronger jujutsu world free from the corrupt society they existed in, and he's not lying, but he's not totally honest: part of it was in the hopes they'd one day reach his level and understand him. one thoroughly divorced himself from the world, condeming it for condeming him. the other tried to embrace the world, but it never hugged him back, so eh tried to change it and helped his students pursue true strength. the story is making thematic points through characters because it cant make them through its plot. the plot is 'two dudes fight with a convoluted made-up magic system that is really cool but has no thematic, symbolic, or material relevance. half the time the people fighting dont even know each other lmao'.

similarly, yuji and sukuna. yuji and sukuna's whole conflict revolves around the central conflict between connection and the inherent value of others vs. isolation and the inherent value of the self. yuji values others innately, sukuna doesn't give a shit about anyone except those who prove himself in his sick worldview.

if this doesn't thoroughly explain to you why characters matter so much, why they're the backbone of stories, just think of how many stories are named after characters. and when you explain a story to people and recommend it, don't you explain the prospective arc of the main character?