In the original story, the Demon Slayer Corps had not known that Muzan had already overcome his weakness to decapitation. Kagaya had blown himself up along with Muzan to create an opportunity for Gyomei, who was standing by near the Ubuyashiki Estate, to behead him.
Unfortunately, the main effect of all that had been to boost the morale of the Demon Slayer Corps' swordsmen. If Tamayo had not seized the opportunity to poison him, that hastily launched final battle might have ended even more disastrously.
From an observer's perspective, the gap between what Kagaya had sacrificed and what he had achieved was far too great.
From the perspective of someone who now counted as his friend, Kanzaki Kumo was even less willing to see Kagaya die young.
Especially now, when Kagaya's children had yet to grow up. Although it was uncertain whether Muzan would drag everyone into the Infinity Castle as he had in the original story, at least some of them would fall inside. When that happened, they would still need him, their master, to coordinate and direct them.
In the end, after Kanzaki Kumo and Gyomei persuaded him together, with Kanzaki Kumo also revealing an appropriate amount of information about the Infinity Castle "kindly provided" by Upper Rank Two, Kagaya finally abandoned his plan to simply blow himself up along with Muzan.
Doma: "I'm not! I didn't! Stop making things up!"
Akaza: "Oh, give it a rest. At least you only became an 'information broker' after you died. People were already spreading all kinds of 'rumors' about me while I was still alive."
Of course, they could not waste an entire courtyard's worth of explosives, either.
Judging by the ability passed down through generations of the Ubuyashiki Clan, which occasionally allowed them to glimpse a fragment of the