If the ancients needed energy and not souls for thier sacrifice, why bother with sacrificing life at all? Surely there's way more aether tied up in mineral resources then there is in the entire biosphere. Or they could build some kind of dyson web and harvest all the celestial aether.

If the concern was to really get enough energy, and not trade souls for souls, then sacrificing the world's flora and fauna is the least efficent way of going about it.