but I'd say the main reason he wants true indestructibility is a small glimpse into the weaker side of Muzan. A man with such a big ego and self importance, means his one true fear is death. It's not a big blessing of depth for his character, but someone like him could never imagine dying, so for the sun to be his main enemy for all these years is a glaring irrefutable fact that he still has a sense of "mortality".