It seems some people here didn't get a few things right:
1. Muzan injected pretty much all of his blood into Tanjiro( although by doing so, he "logically" yeeted that 1000-effect cure into him but oh well, plot armor? ) before dropping dead, although it's utterly stupid how he suddenly became selfless and wanted someone else to become "a perfect demon".
The point is: Muzan is dead, he just turned Tanjiro into a demon.
2. Tanjiro, just like pretty much all demons upon transition, lost his sense of self along with his memories, meaning he's currently just a hungry and aggressive biter.
Nezuko was mentally pretty much an advanced trained dog( with fighting capabilities and eventual flashbacks which only triggered her affection/defensive instinct towards her family ) and we only saw her fully regain her senses/memories after reverting back to a human.
3. The author will probably pull a rabbit out of their ass in some way, but I guess we will have to wait and see. I personally wouldn't mind if Tanjiro lived or died and it feels like the story could end but also could go on.