At the end of endwalker, an ancient was actually brought back. Admittedly this was after zodiark let the souls out, but it was demonstrably possible for someone with sufficient aether reserves and magical capability to resurrect 75% of the ancient population. If only the ascians had a sorcerer of eld with explicitly bottomless reserves of mana on their team. The real question is whether or not zodiark had hotswappable batteries.
