We dont know that. Emet hints at least that the people survived it and we have no idea if they survived with or without their memories intact. It could be that they are simply copies of themselves with less strenght. After all the sundered ascians can retain their memories which means that they have to be somewhere stored. Maybe they lost it all and were like newborns, maybe thought just x amount of the same people with the same memories existed on the shards. After all someone had to remember that time, since we have the cave paintings.
We should also not forget that death still existed even before the whole final days. And at least in the german short story about Azem the people in danger of the volcano are said to be mortals. So who knows maybe not every Ancient was born with a huge life span. Maybe they too had different races with different life spans.
I also doubt that the rejoining would save more lifes because the Ascians plans was not that the survivors of the rejoinings (which would have nearly complete souls at that point) would survive...the only ones alive after the rejoining would be the Ascians and the x amount of Amaurotines that sacrificed themselves. IF they are able to be saved, something that the Ascians cant even be sure about because if Zodiark needed the aether to act then why would these souls still be intact? Honestly for me it sounds more that its the hope they cling to only to have a world at the end, where the source is mostly destroyed thanks to their calamities with only the Ascians as the only ones alive because they find out that nothing can be brought back. (And we should not forget that if Graha and the future Iron works did not change the past, the Ascians would have created a world that even 200 years later was on the brink of dieing...they might have messed it up completely on that)
All of this too without the knowledge if maybe Zodiark truly was just a short term solution and the sound would have been back. At least Venats group believed that he would not be the end solution.
The only question I remember about this that the devs answered was the one posted a few pages back. And there they did not say that the inhabitants are abnormal. They simply stated that the situation being bad would be a matter of perspective and that the worlds are unstable. The worlds being unstable does not mean that the devs believe that the inhabitants are abnormal and deserve to be eliminated. The talk about the perspective is simple too: From the view of the people living now it would be bad because they would brutally die. For the Asicans it would be good because "their god" would be back.
Just a curious question: Would you want them to go the way that we watch billions of lives including us and NPCs we know to be killed? Even at a point were we would not be fractured beings with degraded souls anymore because after the rejoining we would be nearly complete again. Yet we would still be sacrified.