The nature of the Sundering has been spelled out in a Nier Reincarnation side story.

[Lurina]

Not to mention: "In the simplest of ways, they die and die." Given they were devolved to such a primitive state it's likely their life expectancy was short as well, probably dying to things like the common cold and infections from cuts. What a horrific thought.

If we take the description literally, then it makes it sound like people were more or less reduced to the level of infants in terms of intellectual development at first. Combined with the fact that Ancient civilization was clearly global and occupied dense cities, and EE1 saying that early humanity was basically just some scattered tribes, a die-off on a catastrophic scale becomes implicit. It's not like they would have known how to farm or hunt.

Honestly, that argument was always kind of a weird decoy anyway. Because sure, the sundering is murder, but you know what else is murder? Murder. And the Ascians committed a lot more murder.

It's a weird play for equivalency that then refuses to admit equivalency.

Though I'd say that the lead in bodycount seems debatable considering the above (14 worlds, all beginning with mass death), regardless of what other people feel, I've never believed that the Ascians are more righteous than Venat, despite finding their motives more comprehensible. My complaint has always just been that the text is far more sympathetic towards one than the other in a way that comes across as sort of creepy, and ties into the broader messy plotting.

Though admittedly, I've also kinda come to dislike how EW went out of its way to compel you to forgive and like Emet, or more specifically to see the Rejoinings as part of some grander plan that "had to happen" to achieve this happy ending. FFXIV seems weirdly keen to have you stan mass-murderers, and I'm not really here for it.