Hive don't really work that way. There is no 'keeping safe'. All Hive must fight else their worm will devour them. All Hive must prove their right to exist, else they will be replaced by someone under them.
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I see where you're coming from, but I think they (at least the ascendant hive) are intelligent enough to know who they need to keep alive. If what you say is true, why has someone not already taken Crota's place rather than have him revived?
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I think you misunderstand what Ikora means. Omnigul was the one overseeing the ritual to summon Crota back to the material world from his throne. Crota was still alive in his throne-world. All Hive have to fight to survive, it's part of the bargain. They need conquest to feed their worms. Examples of those who fail to feed their worms are on the Dreadnaught, they're where we find wormspore. There are no exceptions, you fight or you die.