Why did ichigo use final getsuga tenshou

In the manga it's revealed that old man zangetsu with the black cloak is actually Ichigo's manifestation of his quincy powers, and the white hollow is Ichigo's zanpakuto spirit and hollow powers together. It also explains why in the final gestuga form he doesn't have a sword anymore, because his zanpakuto is integrated with him, so he doesn't need or have the sword anymore.п»ї

Also this is probably why Ichigo got ALOT stronger before going into the final getsuga where he had the chain around his arm, because him and his zanpakuto strengthened their bond and worked more closely together than ever before. The reason it is called the final getsuga tenshou for Ichigo is maybe because his form is purely offensive and the black getsuga flames are what make up his final form. Maybe the final stage of Bankai is the spirit going back into the shinigami and leaving the sword, integrating with its master, and because the spirit is so strong and the integration is so powerful, maybe after some time the bond breaks and the shinigami loses his powers. Zanpakuto are formed by shinigami slowly and unconsciously pouring out their reiatsu into an empty sword over time, and the zanpakuto is an extension of the shinigami. Here is my theory : The final getsuga is actually the last stage of Bankai where the zanpakuto spirit integrates with the shinigami and both of them become one entity.

Why did ichigo use final getsuga tenshou