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

[Heroman3003]

Actually, this is a fairly interesting idea. A rejoining happens but its physical effects are only seen on the moon from the Source. The 13th shard gets destroyed in the rejoining and all of the souls get funneled through to the Source, but the Source itself escapes physical destruction due to the rejoining occurring on the moon itself. And it eventually results in a side effect of blowing open portals to the moons on the other shards.

Something like this could also explain why Golbez had to wait until Zodiark was gone to make his move (if this is indeed why he is putting his plan into motion now). Zodiark, the Ascians, AND the Watcher would have prevented all previous attempts at rejoinings on the moon, uncertain of how such a thing would affect Zodiark and his prison, even if moon rejoinings may be easier in theory since it's probably the epicenter of the sundering to begin with.

Wasn't a big thing about 8th rejoining that if it happens, all remaining shards also will naturally rejoin because balance of 'sundered' vs 'rejoined' shards would tip too hard?