Requisitioning Obito's Kamui Dimension was not a spur-of-the-moment idea for Hyuga Kagami. He had long wanted an alternate dimension under his sole control.
Now that he had mastered Yomotsu Hirasaka, together with the Eye of the Moon on his forehead, he could mark two or three alternate dimensions for extended periods. Thus, he decided to use two of those slots on Kaguya Ōtsutsuki's Primordial Sphere Dimension and Obito's Kamui Dimension.
He had also given thought to having Obito serve as the warehouse keeper.
For one, Obito had extensive experience using space-time ninjutsu and was exceptionally vigilant when facing enemies who also wielded space-time ninjutsu.
Alternate dimensions were forbidden zones inaccessible to ordinary shinobi, but they were nothing special to the Ōtsutsuki Clan. Therefore, the Kamui Dimension was not absolutely safe and required someone to remain stationed there long-term. As the owner of the Kamui Dimension, Obito was undoubtedly the best candidate.
For another, even though Hyuga Kagami had made an exception and saved Obito, it was not convenient to let him wander around in front of everyone.
Even setting aside the feelings of the other villages, many Konoha shinobi had died in the Fourth Shinobi World War. It was simpler to let Obito conceal his identity and live in the shadows.
As for whether Obito might defect again, Hyuga Kagami was not overly worried.
Obito was not inherently greedy or deceitful. Like Madara Uchiha, everything he had done before had been for the sake of creating an ideal world without losers, where everyone's dreams could come true.
People whose heads were filled with such adolescent delusions usually stubbornly persisted to the end. Just as Obito had previously refused to yield despite countless setbacks, no matter how many blows he suffered, he had remained determined to revive the Ten-Tails and launch