The Reasoning behind The Sundering? (*6.0 Endwalker Spoilers*)

[Nilroreo]

Hydaelyn directly said in her big Endwalker scene that she had faced failures, but was reassured that things would be okay by the stories we told her...

But what failures? She must have already known we hailed from the 7th astral era. If she was already aware that we were as you say 'the one that got closest,' what reason would she have to prematurely declare any of the events leading up to the present as failures? Sure she could've had been filled with remorse over the lives lost as a result of her actions, but to suggest that they were anything but necessary casualties in her pursuit of her real goal is hard for me to believe. The prospect of history going down a different path than what we described to her would've no doubt been scary for her, but as far as were made aware, there were no deviations and everything went exactly as expected.

[Cleretic]

But what failures? She must have already known we hailed from the 7th astral era. If she was already aware that we were as you say 'the one that got closest,' what reason would she have to prematurely declare any of the events leading up to the present as failures? Sure she could've had been filled with remorse over the lives lost as a result of her actions, but to suggest that they were anything but necessary casualties in her pursuit of her real goal is hard for me to believe. The prospect of history going down a different path than what we described to her would've no doubt been scary for her, but as far as were made aware, there were no deviations and everything went exactly as expected.

The failure to stop the other Rejoinings, and presumably other disasters. She knows that if she fails to stop those Rejoinings, the Seventh Astral Era gets closest, but if she manged to stop, say, the Sixth Umbral Calamity, maybe Amdapor was on track to a breakthrough.

The Seventh Astral has a big thing of 'standing on the shoulders of giants', a lot of the leading minds and civilizations are building on departed people; mostly Allag, but others contribute too. But if those giants just didn't die, they'd be perfectly well-suited. Immediately I can look at Allag and say they definitely had everything needed to pull off Plan 'Punch Meteion In The Snout', and I wouldn't scoff at someone saying that Mhach had a chance either.

What I'm saying is that Hydaelyn definitely tried to stop those Calamities, failed, and was reassured by 'well, at least the Seventh Astral Era has good odds'. That's not saying that others wouldn't have had a chance if she'd protected them--and also not saying that later Eras would've been totally lost--but is saying that we are her reassurance. If we die, she doesn't even have a prayer.

[Veloran]

The failure to stop the other Rejoinings, and presumably other disasters. She knows that if she fails to stop those Rejoinings, the Seventh Astral Era gets closest, but if she manged to stop, say, the Sixth Umbral Calamity, maybe Amdapor was on track to a breakthrough.

The Seventh Astral has a big thing of 'standing on the shoulders of giants', a lot of the leading minds and civilizations are building on departed people; mostly Allag, but others contribute too. But if those giants just didn't die, they'd be perfectly well-suited. Immediately I can look at Allag and say they definitely had everything needed to pull off Plan 'Punch Meteion In The Snout', and I wouldn't scoff at someone saying that Mhach had a chance either.

What I'm saying is that Hydaelyn definitely tried to stop those Calamities, failed, and was reassured by 'well, at least the Seventh Astral Era has good odds'. That's not saying that others wouldn't have had a chance if she'd protected them--and also not saying that later Eras would've been totally lost--but is saying that we are her reassurance. If we die, she doesn't even have a prayer.

I'm not sure what plan or breakthrough or odds you're referring to. That is to say, if history up to this point wasn't according to plan - That the seven calamities were failures on her part - then I don't really see what a success state for her is supposed to be. Beating Meteion? As you say there may have been other opportunities throughout history to do it, but she never revealed the conflict to anybody else for them to make the attempt. We then might say that a success state for her was simply Etheirys existing in perpetuity while being protected by Zodiark. But... I don't really see how that's any more of a solution than the selfsame one the hologram meeting in Akademia said wasn't permanent enough.

Part of the reason why it's hard to know if the Rejoinings were actually failures is that she did need them to happen in order to reach this conclusion that is ideal for her. It's hard to argue that Venat was freewheeling it and trying to go for an alternative series of events when everything turned out exactly as WoL described to her and she was actively abetting at least some of it.

[EaraGrace]

Part of the reason why it's hard to know if the Rejoinings were actually failures is that she did need them to happen in order to reach this conclusion that is ideal for her. It's hard to argue that Venat was freewheeling it and trying to go for an alternative series of events when everything turned out exactly as WoL described to her and she was actively abetting at least some of it.

A timeline where she didn’t have to devote the vast majority of her power to maintain the bonds on Zodiark, where civilizations didn’t have to reset every few millennia due to a calamity, and wouldn’t need her strength to resist the Ascians seems much more conducive to her goals.

And I would say it’s very easy to argue it, given we know she was acting to stop calamities. Her support of Tenzen and Ramza, the aid she offered to the various WoL throughout history all point to that end. And then there’s her statements on the matter, which I posted earlier, all make clear her thoughts on how she approached the Rejoinings.

[Veloran]

A timeline where she didn’t have to devote the vast majority of her power to maintain the bonds on Zodiark, where civilizations didn’t have to reset every few millennia due to a calamity, and wouldn’t need her strength to resist the Ascians seems much more conducive to her goals.

Sure. So why didn't she just tell them what the threat was? Even the Ascians would want to see the Meteion ended, and during Allag for example would have had more than enough resources to have a go at her.

Her support of Tenzen and Ramza,

They were both dealing with little-c calamities. That is to say the issues of the Auspices and Ultima appear quite unrelated to the Ascians and the Rejoinings, they were just local issues. And yes she has created WoLs around the time of various Calamities, but by the very nature of our knowledge of them they seem to have a 100% failure rate. If anything their presence had only served to mark Echo-users as portents of doom.

Not to mention her most well-known creation of WoLs by far, on the First, was actually directly causing the Calamity there. And since we now know she's not just a mindless robot dispensing crystals like a slot machine, we can only presume that she knew what was going on and was actively abetting it. Presumably because she knew it had to happen to reach WoL's future, and thus meet her own ends.