All Naruto had to do was K.O. Sasuke a bit earlier in the fist fight, but Kishimoto made sure Naruto depleted most of his reserves and Bijuus during the war and vs Kaguya while Sasuke chilled in the desert. Imagine Naruto doing this in the fist fight:
Or this:
Or retaining 100's of clones worth of chakra.
But it's clear Kishimoto intended for them to lose their arms as some symbolic sign like that heart.
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Or this:
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But it's clear Kishimoto intended for them to lose their arms as some symbolic sign like that heart.