Warning: This theory goes deep into spoilers for the end of MSQ up to the end of Endwalker. Do not continue if you don't want EW and previous expansions spoiled.
Having had a long and pretty drawn-out discussion concerning the narrative riffs and implications on how the primary Ascians were quasi-tempered by Zodiark to continually sacrifice lives to have Zodiark continually restore aspects of their previous lives before the Final Days nearly destroyed Etheirys-that-was, the discussion turned to the idea of Rejoining and rationale behind it as concerned the primary unsundered Ascians. One aspect noted was that aside from the seemingly problematic zeroth attempt at a Rejoining, the disaster that resulted in the Thirteenth being swallowed by the Void and failing to rejoin properly with the Source, each subsequent Rejoining seemed to be more and more difficult and elaborate for the Ascians to realize.
Though the tales of the earliest Umbral Calamities are fragmentary, they all seem to allude to a rather one-sided conflict often styled as "among the gods" that brought about said Calamity. The First Calamity, that of Wind (presumably), seemed to occur without much in the way of opposition and rejoined the Fifth to the Source. Similarly, the Second Umbral Calamity, of Lightning, was caused by having the primitive kingdoms of man engage in feckless wars and increasing industrialization, until it sparked the Calamity and joined Twelfth to the Source. The Third Umbral Calamity of Fire followed a similar pattern as the previous Calamity, except the target this time was the fomenting of conflict between religion and the use of magic, leading to a similar rash of wars that rejoined the Second to the Source. Ditto for the Fourth Calamity of Earth, pretty well documented as the fall of ancient Allag and its warring with Meracydia and the rest of the world - another world war ignited by Emet-Selch causing the Third to rejoin with the Source. The Fifth Umbral Calamity of Frost is a bit of a mystery as to how it started but rejoined the Sixth to the Source. Getting closer to the modern era of FF14, the Six Umbral Calamity of War was sparked by yet another world-spanning conflict, this time between white and black mages of different orders, rejoining the Tenth to the Source.
Finally, we arrive at the modern era and the start of ARR, the Seventh and unaspected Umbral Calamity involving Dalamud, the Primal Bahamut, and Emet-Selch once more manipulating an empire of his creation, this time Garlemald, to foment war and chaos to join the Seventh to the Source. But it is this Calamity where the plans of the Ascians were very nearly upset as the combined might of the Circle of Knowing and the Grand Companies of Eorzea conspired to forestall the actions of the Ascians, their catspaw Garlemald, and work feverishly to stop the Rejoining from occurring. This conflict also illustrates the growing power of the denizens of the Source, now having undergone successive Rejoinings, and granted inscrutable power and endurance that were slowly but surely tipping the scales toward a more even footing even with the Unsundered Ascians, those who possessed the majority of their power as Ancients of Etheirys.
I contend that as of the coming of the proposed Eighth Umbral Calamity, now completely averted with the apparent deaths of all the primary Ascians, a tipping point was reached that made the Ascian's whole plan of Rejoining futile. Essentially, each time a Rejoining occurred that brought the Source closer to becoming Etheirys-that-was, the Ascians essentially stacked the deck against themselves - explaining why paragons of power, skill, and intellect like the Warrior of Light themselves, the Scions of the Seventh Dawn, and Cid Garlond rose to oppose and eventually completely stop the plans of the Ascians. It's shown that the masters of magic and other arts in the Source of this time were even able to achieve feats the Ancients themselves conceded hadn't been replicated, such as G'raha Tia managing to pull the WoL and the Scions to the First across the vastness of the sundered void. Even further, in the now deprecated timeline where an Eighth Umbral Calamity succeeded and rejoined the First to the Source, the successive generations of that war-torn world even figured out how to bend both time and space to send G'raha back in time and across dimensions to become the Crystal Exarch in a long game to undo the Calamity that had seemingly doomed the people of the Source.
I think the Ascians such as Emet-Selch and Elidibus came to recognize that their gambit was doomed as the opposition of the Source to their plans became ever more pronounced, and its denizens more and more capable of opposing and even killing the Ascians permanently. Although their tempering by Zodiark may have rendered them unable to consciously stop working toward the Rejoining, subconsciously they began to recognize that it wasn't going to work, and even more, that they were actively killing individuals and entire civilizations that were growing more and more like their peers and the ancient culture from before the Sundering. Emet-Selch hid this realization behind apathy and despondency, Elidibus by an ever-growing pathological fervor, and Lahabrea through undirected and almost pointless rage. But all of them, deep down, seemed to begin to work to sow the seeds of their own destruction following the Seventh Calamity - something we see most directly with Emet-Selch in the recreation of Amaurot below the seas of the First. He was, in a way, practically begging the WoL to kill him at the end. Unable to face the growing schism at his core, tempered by the primal Zodiark to toil ceaselessly and sacrifice lives to feed the Rejoining, but still aware and cognizant of the growing problem inherent with that course of action. Elidibus, similarly, draws the WoL into a hopeless escalation of conflict that dooms him, even trapping himself in the Crystal Tower, seeming unaware that it was akin to locking himself in a prison and handing the keys to the WoL to close the door on him. Lahabrea continually hops bodies, even taking Thancred's in a childish gesture of spite, knowing that it weakens him more and more until he is finally disposed of.
The conclusion, I feel, is that the Ascians realized the futility of their plans in the same manner that Venat/Hydaelyn did in the time of Amaurot, albeit eons after the fact. The Rejoining was an ultimately futile goal, inadvertently doomed to failure because it slowly but surely created an increasing number of powerful individuals opposed to the sacrifices the Ascians demanded. Each successive Calamity empowered the enemies of the Ascians until a critical point was reached where the still sundered but increasingly soul-dense echoes of the Ancients became an immovable object to the Ascians' designs.