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I'll be honest and say that I'm not sure what the lore is on how Thrall decided on a replacement policy. But it seems like it would be more in character for him to set up a system where existing faction leaders elect a replacement. Thrall strikes me as the type that would take the George Washington stance of not wanting a monarchy or dictatorship.
I'm curious if his decision on this policy was ever explained because it makes me wonder if they just chose it to allow for leaders like Garrosh and Sylvanas.