The Rejoining is a pretty ambiguous term. However, the terms used in German and Japanese seem less so.
Löschung (Deletion / Extinction)
次元圧壊 (Dimension Collapse).
I still find it weird that Hydaelyn has been progressively weakening for fourteen straight eras... but balance needs restoration because we had one good year. What are we using as a metric for balance, here?
And, to play Devil's Advocate for a moment ... think of who Elidibus has been talking to every time he mentions equilibrium, balance, and the good of this star. I'm going to start digging into it right now, but does anyone remember if it was ever to a fellow Ascian - or only to children of Hydaelyn? Minfilia, Urianger, the Warrior of Light... Elidibus wasn't talking to himself on the moon; he was briefing the Warrior of Darkness on his mission. Do we know that we're watching anything but the grooming of a recent convert?
After the recent summoner storyline specifically mentioning how much less energy was required to summon a primal into yourself, I imagined that this was their fallback for FC primal summoning. Someone uses an ability and becomes a temporary anthropomorphized primal for a given time and then releases the energy again, no lasting harm done. But we've heard nothing so who knows where they're going with it.
I've gone over the Ascian scenes over and over and over again, and this is one of the few oddities about Lahabrea's power trip that I think I can explain. It's easy to take his admission to Gaius as, "The only reason we ever concerned ourselves with the primals was to awaken the Heart of Sabik". I don't think, though, that it's meant to be taken that loosely. I think he just meant, "The only reason we let you capture the primals using the Ultima Weapon was to awaken the Heart of Sabik". The legends of the Paragons and their gifts of summoning knowledge go back so far for so long that it seems impossible to not be a big part of their M.O. throughout the ages; far from a secondary goal.
In addition, I don't think it's as simple as "the more primals, the better" but "the more conflict, the better". Seeing as primals drain the land of aether, using them to stoke the flames of conflict is a double benefit. But when one side is winning, that conflict threatens to end. The end of conflict is the singular worst thing for the Ascian agenda. If it takes the loss of a few primals to restore the balance of a raging conflict, so be it. The war cannot end.
Looking at the Allag example, primals were used against the collapsing Allag, Allag didn't collapse because it could capture primals. Indeed, the conflict raged because they could. More and bigger primals were raised, more and bigger weapons combated them. Ultima. Omega. Azys Lla. Dalamud... until, finally, Dalamud triggers the Fourth Calamity.
Looking at a more recent example, the Empire was set make short work of Eorzea until the primals showed up. Ascians ensured that the conflict would be brutal and that all of Eorzea would be caught up in the middle. Except nobody moves ... the Age of Calm sets in. But in that tension, Garlemald makes a move that would and should have triggered a Calamity, but thanks to Louisoix, the job didn't get done.
It's okay, though, because now there's a third faction: Garlemald vs. Primals vs. The Alliance. All against all. But in walks the Warrior of Light, cleaning up the messes. The stalemate is broken; primals are put down, the Empire is put on the defensive. Lahabrea instead moves to awaken Sabik and shatter the "Shield of Light" with the "Hammer of Darkness" ... and Gaius fails. Lahabrea is driven out. The Warrior of Light is too strong.
Elidibus takes interest and backs up Lahabrea. Together, they set the Warrior of Light against more and bigger primals until finally they can move to merge primal and man. They move to the northlands, where Ishgard is about to lose the Dragonsong War. They give the power to Thordan, who will - in theory - start pushing the conflict in the other direction. The Warrior of Light - the Dragons - The Garleans - until the whole world bows to the God King. Surely the Calamity would fall in somewhere along the way - and, if not, they can just give someone the power to combat Thordan.
But now Thordan's dead. What now? Well, the Garleans are still in Azys Lla... and so are the Triad... Good place to start.
Imho, the only thing that breaks the pattern is Elidibus and his talk.
And thus the only thing that matters is whether or not he's lying.