Demon Slayer made it a habit for Tanjiro Kamado, the protagonist, to reach out to a lot of characters and have a positive influence on their personalities and journeys. On the other hand, when it comes to the character of Shinobu Kocho, she is the one example that she couldn't reach, which is one of author Koyoharu Gotouge's best choices.
That is because Shinobu's disdain for demons is entirely justified across the Demon Slayer series because of how Doma took the life of her sister, Kanae. Therefore, while Tanjiro's approach to things is something that pays off for him and his friends across the story, he is also not the best person to try to change Shinobu's mindset when considering how he started his journey for revenge.
Tanjiro didn't reach Shinobu in Demon Slayer, and was the best decision for the latter

As mentioned earlier, Tanjiro Kamado became known in the franchise for being able to inspire and change the mentality of the people around him. It is quite telling that his optimistic nature and determination can shift the tide against Muzan, although it is worth pointing out that he didn't have that kind of impact on Shinobu Kocho, the Insect Hashira.
The story shows that Shinobu’s sister, Kanae, was killed by the second Upper Moon, Doma. This fueled Shinobu's intense hatred for demons, which remained strong until her death during the battle with Doma in the Infinity Castle arc. She passed this hatred on to Kanao during the fight.
Author Koyoharu Gotouge made the right decision by showing that Tanjiro cannot reach out to everyone and that every person has his or her own reason to fight the demons. In that regard, Shinobu is still living her life and helping others, which is something that has to be highlighted in this discussion.
The role of Shinobu in the series

It is worth pointing out that Shinobu Kocho has a unique place in Demon Slayer in the sense that, unlike other Hashira, she was given the opportunity by Gotouge to have a younger sister and successor, Kanao, which makes her a bit more important in the story in that regard. Moreover, her connection with the second Upper Moon, Doma, also allows both characters to be fleshed out during the events of the final arc.
Her mastery of poison also highlights how there are different ways to deal with demons in the series, although it does present the plothole of why other Hashira and members of the Corps, in general, didn't use this tool as well. However, it does serve as a way to indicate the level of intelligence that Shinobu has and the way she can manipulate substances to become a useful fighter, which is an underrated feat in the series.
Final thoughts
Tanjiro not changing Shinobu’s mindset in Demon Slayer was a great choice, as it shows that the protagonist can’t change everyone’s views, adding variety to the story. Shinobu’s hatred for demons makes sense because of Kanae’s death, a loss many people can relate to.
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