Why Rengoku's Death Was A Necessary Evil

Later on in the series, Tanjiro reunites with all the Hashira and studies under them, with the exception of Sanemi Shinazugawa, which is the one challenge Tanjiro will never be able to complete. During this training, Tanjiro learns to master all the Breath techniques, so he's more equipped to defeat the evil Muzan. Had Rengoku lived, he would have been one of Tanjiro's teachers, something they discussed back in "Mugen Train." 

At that point, Tanjiro is already becoming a skilled slayer and knows the Hinokami Kagura, a Sun Breathing technique passed down in the Kamado family. Though it would have been amazing to see these two work together again, the younger man would have learned even more Flame attacks and most likely would have become an overpowered protagonist. Instead, this lack of training allows Tanjiro to find his own path and work hard to get where he needs to be. 

Furthermore, the series stresses how powerful Muzan and his circle are. Though the Hashira are strong, they are still human, which is something Tanjiro reminds Akaza of as he flees the impending sunlight. Rengoku's death emphasizes the dangers the Demon Slayer Corps face every single day and foreshadows that the final battle with Muzan will be just as brutal. Creator Koyoharu Gotouge shows that the good guys don't always win, someone isn't always going to come to the rescue, and that's important to find hope in even the darkest places.