I'm Usually On The Camp Of "Nezuko's Mental Age Regression Is A Bad Writing Decision Because It Takes

I'm usually on the camp of "Nezuko's mental age regression is a bad writing decision because it takes agency away from the sole female main character" - but after Tamayo's explanation that Nezuko traded consciousness for changing her biological makeup to co-exist peaceably with humans, I am forced to admit that as usual any writing decision in KnY I initially hate ends up lining up unexpectedly with the themes of the centrality of love.

The reason Nezuko is such a special demon is because Tanjirou's protection and guidance allowed her to become one.

Most demons focused on the day-to-day of feeding themselves, finding somewhere to rest, and fending off humans who wanted them dead. Nezuko is a rare demon who had the protection of someone else, so she could afford to sleep most of the time and change her biology. Only Nezuko could afford to trade so much of her consciousness away and to become so childlike, airheaded and defenceless because Tanjirou took care of the day-to-day for her. Unlike the other demons, she didn't have to always be on guard and live with an everyone-for-themselves mentality.

Nezuko was also able to not taste a single drop of blood because Tanjirou carried her away from their family's bodies (while trying to get her to a doctor) and was strong enough to hold her off until she got a grip. And only she lived past being discovered by a demon slayer, because Tanjirou defended her and she defended him. And when she did feel the urge to eat (Red Light District arc), Tanjirou was there to stop her.

Also very importantly, Nezuko's love for Tanjirou provided her a very strong motivation to stay in this sub-human state of consciousness for the long period necessary.

Most demons did not have a very strong reason to force themselves not to eat. Either they had no respect for humanity to begin with (Gyokko, Muzan, Spider Demon Ryu), or everyone they loved had died and they were subsequently in too much despair to put up a fight (Tamayo, a certain spoiler demon who was so suicidal that forgetting his loved ones ironically helped him live longer).

Meanwhile Nezuko really needed to return to being human, because her brother will not stop fighting demons otherwise. Regaining consciousness and convincing him to quit isn't an option for her because the demon slayers wouldn't let her live, and Tanjirou won't let her die. (And if he did, he would never recover from her death.) Like I don't know how much Nezuko thought this through, but Tanjirou's plan to shake information out of demons while trying to kill them is frankly terrible, while Nezuko's course of action to just change her biology was wildly successful lol. She had the better idea. Just as Tanjirou walked this long road for Nezuko, Nezuko did for Tanjirou.

Man, this is why I really love KnY. When the narrative said that nothing could break the bond between Tanjirou and Nezuko, when it said that love lets people exceed any reasonable expectation and achieve the impossible, they weren't just saying it. Everything has always been there, lying out in the open and waiting for us to realize it. Like, which other demon could spend so much time sleeping away so that they can change their biology. Only the little sister whose big bro is still alive and willing to take care of everything for her, and who herself loves him enough to achieve the impossible, of course.