Tanjiro Kamado is a great hero for Demon Slayer to follow, but the series would have been drastically darker if it focused on one of the other Hashira: Sanemi Shinazugawa. The two characters have remarkably similar backstories, but how they handled their tragedies differs greatly. Sanemi Shinazugawa is much like a twisted reflection of Tanjiro -- had Demon Slayer starred him in the lead role, it would've been a completely different series.
Sanemi Shinazugawa grew up as one of seven children to an abusive father. Their father was killed one day after someone with a grudge stabbed him. Sometime after this, their mother became a demon and killed five of his six siblings before Sanemi was forced to kill her in self-defense. He became partially colorblind after this attack, and ran away, leaving his only remaining sibling, Genya, all alone.
Sanemi then began hunting demons down in a suicidal attempt to destroy them all. At some point in his hunt, he met Masachika Kumeno, who became his friend and mentor as he trained to become a Demon Slayer. However, Kumeno was killed during a fight with a demon, leaving Sanemi the only one to become a Wind Hashira. He holds a deep hatred for all demons as a result of the destruction of his family and the death of his friend.
His story mirrors Tanjiro's in many ways, with both losing their entire families with the exception of one sibling. Both were also the eldest sibling of a large family and felt a sense of responsibility for them. The difference is how they handled their trauma and loss. Tanjiro stayed by his sister's side despite her demonic nature, while Sanemi abandoned Genya to seek out vengeance.
Had the series followed Sanemi instead of Tanjiro, it would have followed his journey of anger and hatred. It would have explored his quest to destroy all demons, as well as his ascension to Wind Hashira. Sanemi's battles against demons would have been visceral and numerous, more so than Tanjiro's battles as he would hesitate before finishing off a demon.
Sanemi's story is full of death and tragedy, and because of his trauma and wish to protect his brother, he turns him away and does everything he can to force Genya out of the Demon Slayer Corps. He even goes so far as to try to mutilate Genya's eyes in an attempt to blind him and make it so that he's unable to fight. The emotional turmoil Sanemi felt during his journey would have made the series a darker examination of how people turn towards revenge when they feel they've lost everything. Between his family dying, killing his own mother and losing his friend and mentor to a demon, Sanemi's faced more loss than most people.
Tanjiro's story is equally as tragic, but Sanemi's story would have put the series in a much darker place simply due to how he handled his loss. He went on a rampage against every demon he could find, not caring if he was killed in the process. His is a story of how anger can fuel a person to become self-destructive if left unchecked.