@Minamu135 this and this
@IlsaTilsaKilsaMilsa pretty much.
I didn't realize until I reread this, but Tanjirou was DEAD. Muzan "revived" him by turning him into a super demon. You know the type that had Muichiro die along with Genya. Even If Tanjirou was recently turned, he should have been quite the challenge (basically I dont think anyone should have survived his rampage If we are being realistic).
So my question is: If he was dead, and gets turned back into a human, shouldnt he remain dead? He never got to eat human flesh so he really shouldnt have a lot of fuel to heal himself. And we are talking a lot of blood loss among the injuries.
I know the answer is "because is a shounen manga", but that's just a sad justification... Shingeki no Kyojin IS a shounen manga yet look where it went! Saying is "a shounen manga, is allowed to be shitty..!!" Is a shit excuse tbh.
but then again we are talking of kny fans so...