I really wish Kokushibo had met Tanjiro. Even if it was only just to talk. To meet a family who did take on sun breathing. Their family didn't die out; through Tanjiro their techniques lives on and so does the memory of them.
Yoriichi was right:
"Someone who could very well surpass our talents may be a crying baby at the moment".
"They are going to arrive at the same place regardless".
Regardless of strength and the skill difference the new generation and a new sun breath user succeed where they failed. They defeated Muzan, eradicated the demons with him, and "reached the end of their lives feeing ease".
This was such a beautiful moment but a sad way to pass for Kokushibo. Akaza really was the only upper rank who died like a warrior. He and Kokushibo both regrew their heads and surpassed all weakness except for the sunlight. But only one died with a smile and the other died unsatisfied with his loss and his life leading up to it.