(Wildly speculative) idea about the next overarching threat - *spoilers*

[Iscah]

In the Heavensward post patches, you speak with Minifilia/Hydaelyn who has now become the Word of the Mother, and here she explains that she has failed to stop the 7 previous calamities/rejoinings and you can't let them do it once more because she won't have the energy. So the basic idea is in place pretty early in the story, granted not as fleshed out as we come to understand in Shadowbringers.

So, even putting aside Cleretic's objections to your post, the issue is entirely about Hydaelyn's strength and nothing to do with it being a turning point for Zodiark's reconstruction. And we still could have hypothetically found some way of recharging Hydaelyn, like Urianger did by bringing us together with the Warriors of Darkness to offer our crystals.