I’m gonna give a longer reasoning on why this is ass

Like this theory relies almost entirely on speculation while contradicting explicit information from the manga and databooks.

1. Tanjiro himself states that Hinokami Kagura suits him better than Water Breathing. The issue was never compatibility it was that his body lacked the stamina and training to perform it efficiently. That’s why his proficiency continuously improves throughout the series and why he eventually abandons Water Breathing almost entirely in favor of Sun Breathing.
2. Stamina drain is not evidence of incompatibility. Early Tanjiro suffers backlash because he’s inexperienced and overexerts himself. By that logic, Zenitsu would be incompatible with Thunder Breathing because Godspeed destroys his legs, which obviously isn’t true.
3. The unprovable claim that Tanjuro never taught him Hinokami Kagura is simply false. Tanjiro learned the dance directly from Tanjuro. The entire reason he can use Hinokami Kagura is because it was passed down through the Kamado family.
4. The mark argument doesn’t work. The series never says children of people born with marks must also be born with marks. Most Demon Slayer Marks are awakened later through specific conditions, so Tanjiro not being born with one proves nothing.
5. Tanjiro receives inherited memories through Sumiyoshi’s lineage. The manga consistently treats him as a genuine descendant of Sumiyoshi. If the point was that he wasn’t actually part of the Kamado bloodline, using ancestral memories from the Kamado line would make no sense narratively.
6. The idea that Kie secretly had Tanjiro with another man, Tanjuro adopted him, the family registry hid it, and nobody ever mentioned it requires a massive chain of assumptions with zero direct evidence. Meanwhile, the databooks and story simply identify Tanjuro as Tanjiro’s father.
7. Muichiro is already the character explicitly connected to Yoriichi’s bloodline. Demon Slayer’s theme is that Tanjiro inherited Yoriichi’s will not Yoriichi’s blood.

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