Perhaps the writer of the fanfiction is trying to create a different character arc for Harry. Maybe he has faced so many hardships that he becomes disillusioned. For example, if he has lost too many loved ones and feels that his efforts are in vain, he might stop trying. It's a way to add a new layer of depth to his character.
One possible reason could be that he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
One possible reason could be that the author wants to create a sense of mystery. By making Harry Potter stop talking, it makes the readers wonder what he is thinking or feeling, and adds an air of enigma to his character. Maybe he has witnessed something so shocking that he is rendered speechless for a while.
The plot takes a darker turn. Without Harry's determination, the fight against evil might seem almost lost. His friends would have to scramble to fill the void he leaves. For example, Hermione might become the main driving force, but she would face many challenges without Harry's leadership.
One reason could be a misunderstanding. Maybe Hermione does something that Harry misinterprets, like getting too involved with her studies and seeming to neglect their friendship. Another could be outside influence. For example, if there are other characters who spread false rumors about Hermione to Harry, it might drive a wedge between them. Or perhaps Harry starts to develop different values and beliefs that clash with Hermione's, leading to a breakdown in their friendship.
Well, perhaps Harry could use his magic skills like the Patronus charm to hold Loki at bay. Since Harry has experience dealing with dark forces, he might be able to outwit Loki's trickery.
Perhaps Harry has grown tired of always following Dumbledore's instructions without fully understanding the reasons behind them. He might want to make his own decisions and take control of his own destiny, rather than relying on Dumbledore all the time.
Perhaps Harry teams up with some of his friends from Hogwarts. Ron could use his strategic thinking, and Hermione her vast knowledge of magic. Together, they create a magical trap for Ebony Maw. Harry, being the main hero, would be the one to lure Ebony Maw into the trap and then use a powerful combination of spells to stop him.
Maybe Harry Potter could stop Thanos with the help of his friends. Hermione's intelligence could come up with a plan. They could use time - turners to go back in time and prevent Thanos from getting the Infinity Stones in the first place. Ron could be there for moral support and also use his chess - playing skills to outwit Thanos in some strategic battles.
One possible reason could be that the fanfiction author wants to focus on other characters. Maybe they feel they've explored Naruto's character enough and want to give more spotlight to characters like Sasuke or Sakura. Another reason might be that the plot of the fanfiction takes a turn where Naruto's acting skills are no longer relevant or needed.
One reason could be that Harry wants to preserve the integrity of his real story. Fanfiction often takes the original story in different and sometimes wild directions. He might worry that it could distort the truth about his adventures, his friends, and the sacrifices made. For example, some fanfictions might create romantic relationships that didn't exist or change the motives of characters, which could mislead those who are new to the real story of Hogwarts and his battles against Voldemort.