I believe in 1.0, prior to Dalamud's fall, Atamos (plural) attacked Aetherytes around all of Eorzea. Not sure why. It seems likely to me that since Colonial Sharlayan was abandoned that no one protected them, or replaced them when they broke. I also think the idea that they just crumbled over time holds merit as well.

Typically when characters speak of, 'Another Star,' they're speaking of one of the 13 Reflections of the Source. We're on the Source, and the 13 Reflections are other versions of the world we live on. The 13th reflection (or Star) fell to darkness and became the Void. It seems likely that is the Star Midgarsormr came from.