If the Soul King falls, the rest of the world may fall, too.
At this point in the story, protagonist Ichigo Kurosaki is at his strongest, just in time to face incredible new foes like King Yhwach and the Schutzstaffel. In cour 2's climax, those elite Quincy suffered an apprent defeat, but there's little doubt they'll be back for more, and the Soul King himself is what's at stake. If the Soul King falls, the rest of the world may fall, too.