Well, as we can see in the latest chapter, Ichigo has a very hard stand against Yhwach who assumes to see all the possible futures and transform them through his will. By e.g the transformation of the Soul Kings palace, he has proven that he is not joking about his powers. But there has to be something like a blind spot or otherwise no one could stand a chance against him. I could imagine that Haschwalth might be part of this blind spot. As I said above, Yhwach assumes to see every possible future and can change it to his will but how can a being with self-proclaimed "absolute sight" be really sure that he is not missing something ? Just because you can't see or sense something doesn't mean it doesn't exist or could exist. Like a sand grain which is hidden by the countless other grains Yhwach is watching if you want to use the metaphor of the Quincy king. And even if he is not missing anything might he be arrogant enough to assume that there is a possible outcome which is so unlikely that it doesn't need his full attention ? Back to Haschwalth: What if this outcome is the defeat of the Grandmaster of the Sternenritter which Yhwach did not foresee or let slip through his attention ? Yhwach called Haschwalth his "other half" and well, if you lose your other half - that hurts. And his death might even have more impact on Yhwach: Normally, "The Allmighty" changes his possesor at night. But if Haschwalth would die before the next night ... where would "The Allmighty" go ? Would it vanish ?