The Indigo Nichirin Sword is representative of the beast like actions of Inosuke (Image via Demon Slayer) Inosuke ... cut through the Upper-Moon three's head and injured him heavily.

Rengoku's survival would have changed the trajectory of the story, altering key events and the Demon Slayer Corps ... Zenitsu, and Inosuke further. He could have been sort of a mentor to them ...