I like muzan. The fight was kind bad. But within his manners, behaviour if im wording it right.
He had disdain for humans, at least thats what i got from his actions on the manga.
So him fighting like he did, just flailing his monster arms around, underestimating everyone was within his character.
Because of that and he saving his OP demon arts as a last resort he lost.
(Nevermind the 3k effects from tamayos drug).
I'm fine if it ends like this. With sanemi ( sanemi deserves to be happy somehow) and giyu surviving. Realistic speaking on fantasy manga about demons and ultrahealing powers, that's okay. Some people dying. And tanjiro dying is okay Too. The manga is kind sad most of the Times, I hope it finishes the same way. With sadness embracing everyone, even thought they will be happy that no more human consuming monster are coming the way.
There still time for muzan/tanjiro merging while they were together. That kamado blood is the one that can resist the Sun if it is mixed with demons blood. (That would suck so much and be cheap asspull)