The numerous A-Rank, B-Rank, and below Hunters were working extremely hard to prevent something as disastrous as mass Gate Outbreaks from being realized. However, there was an undeniable limit to how effective they'd be up against the Monsters from High-Red and Purple-rank Gates.
It was for these reasons, and the ones concerning the danger of following Cassius, that the Japanese S-Rank Hunters assisted in the Gate defense efforts. If anything, they had to buy enough time to see if Cassius could clear the anomaly that was the World's End Gate.
In the possible scenario where Cassius was unable to successfully clear the Gold-ranked Gate, well… at that point, all hope would be lost for humanity's continued survival. Therefore, nothing they did would've made a difference anyway.
But they had to try…
As the last few S-Rank Hunters in the world, they owed it to themselves to do what they originally became Hunters for in the first place.
To protect.
