maybe...because Venat met us at Elpis and we told her what happened with our world, all the shards, where we are from, about Hydaelyn, etc. So, Venat could have just make things in order to keep our existence in her future possible (by sundering Zodiark and becoming Hydaelyn) since we and she was the only ones to know exactly about Meteion and the Final Days's source. If she changes history, our existence or knowledge could be neutralized or worthless.
About Zodiark "restoring the world" I understood that it was impossible. Zodiark is a Primal and the top most aether consuming at that. In order to keep him you have basicaly to destroy the life in the world. All creations must have to be given to him as "sacrifices" from time to time to keep him existing, as we learn in ShB. So, the very existence of Zodiark in it's full form is a threat to the planet itself. The sundering could make him weaker in a way that he could not be summoned anymore (prision at the Moon) and sacrifices made to him not more possible (that's why the unsundered tried to Rejoin all reflexes through Calamities in order to give Zodiark power enough to free himself).
I think these 2 points (our very existence that Venat met at Elpis and the necessity to weaken Zodiark's power before he consumes all the world to keep existing), are reason enough to justify the sundering instead of doing something else ( but I agree about trying to use Zodiark to kill Meteion before the sundering).