The LK was pretty vulnerable. He was even vulnerable against plague bomb. If plague bombs were able to injure him then the Aspects's power would destroy him. DW for example was nigh unkillable except with the immense power of the Dragon Soul. The LK was injured by merely plague bombs. He was pushed back at the light's hope. He certainly could not just rip souls out of anyone or everything without certain conditions. He would just rip Tirion's soul out at the Light's Hope if he could because he purposely sent the Ebon Blade of the suicide mission to lure Tirion out.
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Again why would you assume that the LK would last long enough to pass a certain threshold of power? The LK is certainly not too powerful.