Eorzea has much Light? How?

[MrThinker]

Their ultimate goal is to resurrect Zodiark, then have him fuse with Hydaelyn back into the (supposed) primordial dawn / twilight god(dess), which would indeed keep things balanced.

The Ascians would probably tip things back toward Light on a Shard world (or the Source) if they absolutely had to; Dark-flooded worlds are useless to them (see the Void).

No they don't want Zodiark to fuse with Hydaelyn, they want Zodiark to subsume Hydaelyn, and have him be the only one left.

Something which makes sense since they are both apparently primals. Garuda tried to eat Ifrit and Titan, Thordan tried to eat the Warring Triad, and I think Bahamut absorbed the aether of defeated primals somehow when Dalamud was falling, or that was what I was led to believe at any rate.