Kishimoto does remember what he wrote from Part 1

[Darth Uchiha]

@Eight Gate Opening Part 2
Ya know, Goku can use Kaioken to make himself 100 times more powerful, at the price of pain. WITHOUT DYING. And everyone can do that, not just Saiyans, meaning if only Gai can open the gate at this point and also at the price of his life unless something supernatural happens (Naruto), then he will die.

THRED SOLOD

Actually, kaioken will kill it's user if over used.