Zodiark is never credited with creating life as far as I can tell. He is credited with regenerating the physical world so life could live on it after he stopped the Last Days. Then the Convocation of Thirteen decided that they would let the life on the world continue to develop until there was enough aether to sacrifice to Zoidark to get the 75% of the Ancient population that was sacrificed to Zoidark back.

Never mind that so far there has never been any mention of Zoidark (or any other Priaml for that matter) releasing back the aether they have already absorbed until they are killed. And given that in order to summon Zoidark it took the aether of the 50% of the Ancient population... I have a hard time seeing anything giving Zoidark enough aether at par with the Ancient's aether to keep him going and get the Ancients back other then the aether of other Ancients. Or at the very least the aether of other sentient life.

No matter what is getting sacrificed to Zoidark, there seems to be the start of a cycle of "sacrifice enough aether to be most of the population of a planet to get something from him". With the "something" people are getting from Zoidark being anything from rewritten laws of reality to a restored physical world to the souls of people already sacrificed to him. Where does that kind of sacrifice system stop?

Or to quote someone else in the game on the topic of people being dependent on (false) gods...
In Eorzea, the beast tribes often summon gods to fight in their stead─though your comrades only rarely respond in kind. Which is strange, is it not?
Are the “Twelve” otherwise engaged? I was given to understand they were your protectors. If you truly believe them your guardians, why do you not repeat the trick that served you so well at Carteneau, and call them down?
They will answer─so long as you lavish them with crystals and gorge them on aether.
Your gods are no different from those of the beasts─eikons every one. Accept but this, and you will see how Eorzea's faith is bleeding the land dry.
The subjects of a weak ruler must needs look to a higher power for providence...and their dependence comes at a cost to the realm.
The misguided elevate the frail... And the frail lead the people astray.
Unless a man of power wrests control...the cycle will never be broken.
You... You of all people must see the truth in this. You who have the strength to rule...
Substitute Eorzea with Amarot, the Beast Tribes with the Convocation of Thirteen and the Eikons with Zoidark and you pretty much have Gaius describing exactly what the situation was after Zodiark was summoned is. Complete with other people deciding the cycle had gone on long enough and breaking it...