I think I can cover this one.
Zodiark was understood to be the evil big bad as much as Ascians were understood to be servants of the big bad.
This all changed in Shadowbringers your example of peak storytelling.
You see in Shadowbringers we learned that both Zodiark and Hydaelyn were primals born from the sacrifices of the ancients.
Zodiark was specifically created to forestall final days with his amazing power. Zodiark is successful but is also a gigantic ravenous monster tempered many remaining ancients.
The Plan the Ancients had next was to have yet another mass sacrifice to fully cleanse the star and to bring every ancients back.
Hydaelyn is created instead sealing away Zodiark and sundering the Star creating the shards as we know them today.
So Zodiark and Hydaelyn are not the penultimate forces of Good and Evil. Whatever created the Final Days the Ancients did not know except for a sound "A cacophonous keening from beneath the Earth" as said by one of the spectred ancients in Amarot - A sound which distorts all living beings within earshot turning them into blasphemes. This sound is the Endsingers song.
Yes, the Final Days is a byproduct of the hubris of Ancient Society and one rogue Ancient going a bit nuts.
Hydaelyns creation was in part to make sure the mistakes of the past are not repeated, though people's shattered existence will be made to suffer in life, it will enrich their purpose such as creating an answer which may overcome despair.
Given so many lives are sacrificed in the process to make both Zodiark and Hydaelyn, the fact that many ancients were tempered into Zodiarks thrall in general, whatever power Hydaelyn has which seals Zodiark away... You could not reasonably say there was an opportunity to tell people there's a blue bird lady at the edge of the universe somewhere lets go get her that'll fix everything!