I don't necessarily think summoning Zodiark was a bad idea. I think it was one of the many "we had no choice" cases in the game.

However, the story was dead-set on showing us that those who cannot confront despair will unmake themselves when given the tools to do so, and that's exactly what Zodiark was. We were shown every indication that the Ancients would continue to sacrifice their people to Zodiark in the face of their suffering, which was now an intrinsic part of their world after the events of the Final Days. The first summoning of Zodiark was necessary. The second one? Possibly necessary, but more ambiguous. The potential third one, however, was a clear and obvious indicator that the Ancients had no idea how to cope with loss and were incapable of doing so. The writing was on the wall. They damned themselves to oblivion the moment they had the power to sacrifice themselves in pursuit of what they once had. Zodiark had to be stopped, and the Sundering was the only means of doing so due to how much stronger Zodiark was than Hydaelyn. I don't think it was Venat's intention all along, either. Unless I'm remembering this incorrectly, she explicitly rebuked that after our fight with her. It's much more likely that it was a desperate, last minute decision made by someone who was every bit as traumatized and terrified as the other Ancients doing whatever she could to give hope to the world.

Because there sure as fuck was no hope where they were at the time.