The Ancients still deserved better, after everything

[KageTokage]

You basically summed up part of why a sizable amount of people weren't very happy with how Endwalker played out (Which has been discussed at length in topics both in the lore forum and the the general discussion).

After both Amaurot and Mare Lamentorum try to get you to empathize with the Ancients, for them to suddenly paint their civilization as being deserving of destruction through calculated means as opposed to an accident or something more easily forgivable felt...awkward.

Are they really destroyed though? As long as the endsinger didn't take your soul, you were reborn. It's heavily implied that the people in the source and the shards are just reincarnated into less aetherially dense forms. Technically the Ascians still live, they are just sundered. They might not remember it, but they possibly will after each return to the life stream. It seems to be when people regain memories based on hints in Endwalker.

People oft refer to their fate as genocide because it can also describe the death of a culture. Even though they technically persist in the form of the sundered races, everything they once were ceased to exist the moment the Sundering happened.

The whole deal with regaining memories only applies to those who've had them blocked out through means magical or otherwise because the Lifestream specifically tries to cleanse souls and render them a blank slate. It's only the "scars" left from traumatic memories of the Final Days or whatever manner of extremely powerful memory alteration Kairos utilized that persist through multiple lives.