Originally Posted by
Absimiliard
Siphoning wasn't even part of Zodiark's design. At its core, it was little more than an ultracomplex application of the creation magics utilized by ancient society on a daily basis. The aether consumption shown in later primals was intentional; the Ascians taught people flawed derivatives of creation, which in turn begat flawed creations.
The intention itself had nothing to do with it. It was how Zodiark enacted this intention that made the difference. What protected the world was an enormous shield of dark aspected aether, which Zodiark maintained throughout the ages. Aether and dynamis are for the most part incompatible, the former being quite detrimental to the latter. That said, it's been proven a sufficient amount of dynamis can overcome this deficiency and act upon aether without being undone in return. We don't really see it touched on too much in-game, but there is an implication the shield would have failed eventually. No way of knowing when, of course. Probably thousands more years.
It did in fact care for the souls within it. One of the conditions set when they created Zodiark was to safeguard those souls until such time as they could be brought back. The ancients had no intention of leaving their friends and loved ones in limbo forever. Additionally, those souls were in part responsible for Zodiark's instincts. Absent a heart, all it had left to guide it were its programming and the collective desire of those souls to see the world saved.