I'll try to write out a summary explaining my thought process on this feat:
From Novel 2: "However, he had heard Yhwach’s goal was not to rule over the three worlds but to remove the boundary itself between the worlds and return everything to primordial soup."
From Novel 3: "Had he not defeated Yhwach, the boundaries between the three worlds would have disappeared and we would have returned to a universe in which there would be no cycling of konpaku, wouldn’t we?"
From the manga: "The living world. The Soul Society. It will all lose shape and become one when faced with my power."
We have explanations from earlier in the manga from Rukia (
confirmed by Urahara) and Hidetomo that ties into a "universe with no cycling of konpaku".
Here is the explanation from Rukia,
page 1,
page 2 and
page 3.
Here is the explanation from Hidetomo,
page 1 and
page 2.
If the two worlds, Soul Society and the Living World were to mix, then everything would collapse. The "collapse of the Universe" as Rukia and Urahara put it. Life and death mixed together, which is exactly what
Yhwach described as what would happen.
Life and death, the Soul Society and the Living World are kept separated by the Dangai, according to Hidetomo. So it seems reasonable that if that boundary no longer existed, then the balance would be disrupted and both worlds would lose shape and collapse into one. Meaning if Yhwach was to remove that boundary, then his plans would be fulfilled exactly as he describes.
People say "
the plural 'boundaries' was used, therefore Yhwach had to be destroying something else" but Tokinada doesn't say Yhwach was going to
destroy them. He simply said that if Yhwach won, then the boundaries would have disappeared. This is something that would have happened anyway with the 'collapse of the Universe'. Even if we did consider the Garganta to be a "boundary", it doesn't mean Yhwach had to personally destroy it for it to disappear.
People say "
Yhwach was going to destroy the realms using his AP" but Yhwach doesn't specifically say he will destroy the Soul Society and Living World; he says they will lose shape and become one, which is exactly what would happen if the boundary (Dangai) between the worlds was destroyed by his "power". The end result, is that they will be merged, not that he will annihilate them so that there is nothing left.
People say "
but Hueco Mundo was going to be destroyed too, and it isn't connected to Dangai", but that isn't relevant. Hueco Mundo isn't connected to the Living World or the Soul Society or the Dangai, but it would still get destroyed in the event of a Universal collapse because
everything would be destroyed. According to Yhwach if life and death were to mix, then Hueco Mundo would be mixed in too. So if the Soul Society and Living World collapse, then Hueco Mundo is collapsing too.
People say "
we saw Yhwach's Reiatsu coming out of Soul Society, so he meant to destroy Soul Society itself". Maybe he did. We can only speculate as to exactly what would have happened if Yhwach hadn't been interrupted. Maybe Yhwach was going to destroy the entire planet as part of him destroying the Dangai, maybe he was just personally erasing the Soul Society before he was going ahead with starting the collapse of everything. I don't like speculating on something we can't actually see happen.
My overall point is that trying to extroplate from this that what Yhwach really intended to do all along was blow up the Garganta is too speculative for me to accept as the primary justification for Yhwach's ratings. The Garganta isn't called the boundary between worlds, Yhwach never claims to want to personally destroy the Garganta. We have alternative explanations that fulfill the "life and death mixing together" part of Yhwach's plan.
