Thought this too, but losing her to Muzan pretty much means gameover. However, changing her back to human would set Muzan back hundreds of years in case they all fail to beat him now. And as far as fighting capability goes, Nezuko is strong, but still not as strong as a pillar; she would inevitably get captured before her potential as a demon hunter is realized.
@toastedguy She is not that strong anyway. I still feel her ability is lower than a lower moon demon. As a demon, she lacks the skill to become an effective demon hunter. Her thought process is like a child and after she used her power, she pretty much slept for days so no more useful than a regular hunter.
I get there he feeling that Nezuko will go rogue and Tanjiro is gonna have to behead her later on. They’re not just gonna hint at her being capable of losing herself like they did versing Gyuutarou and Ume without it becoming something that’s happens in the future again.