During the climactic battle of the Mugen Train arc, an Upper Rank demon offers Rengoku the opportunity to become a demon. Why didn’t Rengoku become a demon?
Rengoku didn’t become a demon because of his viewpoint on humanity. He says that human-life has value and nobility because your time on Earth is limited. Growing old and passing away is a natural part of the human experience that becoming a demon would ruin.
Akaza’s main interest in Rengoku stems from an appreciation of the fighting arts. He wants Rengoku to become a demon so they may spar for all eternity at full strength. Yet, Rengoku would rather not reach his full potential so long as he can remain a human.
Rengoku explains why he didn’t become a demon
In chapter #63, Akaza, an Upper Rank demon, shows himself to Rengoku and Tanjiro. The Demon Slayers were able to defeat the Mugen Train demon and prevent civilian casualties as the vehicle ran off the tracks.
Akaza, who approaches the crashed train after receiving instructions from Master Muzan, confronts Rengoku.
He asks the Flame Hashira to become a demon. Akaza says he can sense Rengoku’s incredible battle sense and fighting spirit that has been honed to near perfection. Since Rengoku is a human, he is incapable of reaching his maximum potential.
Akaza continues to explain that humanity is ultimately dragging Rengoku down. The Flame Hashira will grow withered and eventually pass away, whereas if he becomes a demon, he can continue to hone his skills for all eternity. Even the Demon Slayer corps will replace Rengoku in time.
Rengoku refutes Akaza’s ideology. He states that growing old and dying is what gives beauty and meaning to the fleeting span of a human life. Human lives have value and nobility, Rengoku says, precisely because they age and eventually pass away. He finishes his retort to Akaza by saying they value different things, and he has multiple reasons for not wanting to become a demon.
So that’s the answer. Rengoku didn’t become a demon because of his viewpoint on humanity. He says that human-life has value because of its limited life span, and becoming a demon would remove the value and nobility of living your limited life to the fullest.