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

[Lyth]

It's literally showcasing that the Ancients could be physically immortal if they wanted or needed to be, but that they still don't consider themselves above what's supposed to be the natural cycle of their planet.

Are the Ancients truly immortal, though?

When you meet Hesperos, he brags that becoming a Hemitheos brought him immortality:
'For once, be grateful to witness a feat beyond your meager abilities. Beyond your shallow reckoning. I have slipped the surly bonds of mortality! I have become one with our mythic creations, and rid myself of imperfections. I stand upon the threshold of godhood! As a hemitheos shall I serve Master Lahabrea, ever faithful, until the end of days...'

Stock villain speech aside, it's a bit of a strange boast to be making in the company of a couple of other supposed 'gods'. If the Ancients are all immortal to begin with, then why is that so special? Why does that quality make him, specifically, a 'demi-god'?

It's also strange to use euphemisms around death in a society that doesn't have to worry about it. Hermes rightfully points out that despite his predecessor's achievements, there's still so much that they could achieve by continuing to live:
'It's just... I cannot fathom why someone so great and wise, who could still do so much good, would want to end it all...'
And he has a good point. You could spend lifetimes upon lifetimes in the single-minded study of one subject, and not run out of things to learn about and discover. It's also not surprising that Venat in particular, even in retiring from her role as Azem, never ran out of things to learn about and be passionate towards; there's always something new and exciting to find over the next horizon. I don't think that you can make a definite call either way without some hard lore-based evidence.

[Vyrerus]

Are the Ancients truly immortal, though?

If they wanted to be, yes! <--- The keywords in what you quoted.

You listed two examples in the your post. It's considered unnatural and bad to their society, though. But look how relatively easy it is for them. Venat becomes Hydaelyn, and goes on living for 12,000 years + however many she lived prior to that in Ancient Etheirys.

Emet-selch somehow remains Unsundered, and then through dickbag means continues to live for 12,000 years + however long he lived before in Ancient Etheirys.

As far as Hesperos goes, it's more or less him saying that he found one of the ways an Ancient could become immortal. His method is even more vague than either of the ones we've seen firsthand.

As far as them talking about death in, "Euphemisms" embrace the Bacon Necktie, Lyth. It's not a euphemism. They literally don't die when their bodies do. Their souls go on living and come back in new flesh. Hell, Hades and Hythlodaeus "die" in the modern age, and are given a true chance at revival in Ultima Thule, but opt out of continuing to live.

As far as the ole Hermes denial? That's just a riverfall in Elpis, babe. He doesn't want to admit that choosing to return to the star is a good thing for society at large. New minds get to move into new positions, and have the pressure of bearing society's woes and weal. It lets them grow as people. If everyone in their society chose to continue living for however the hell long they can keep themselves going, then it'd always be the same minds taking care of everyone. The youth would grow up bridled by indolence, and the society would stagnate. I mean, look at how bad it is in our world where politicians don't live forever, but either have no term limits or life sentence terms OR prolong their agendas by raising their children into them.

Not too mention the fact that couples can go out together. You ever watched someone lose the love of their life, and then live on in shambles afterwards? It ain't pretty.

[Kordarion]

If they wanted to be, yes! <--- The keywords in what you quoted.

If an ancient has to become a primal, merge themselves with a mythic beast or cheat death by going to an in-between world when they should be returning to the aetherial sea to become immortal it doesn't mean they are naturally immortal. If a modern human found a way to become immortal that only affected themselves you wouldn't go around saying that all modern humans are immortal would you? To be fair whatever it is that the unsundered did to return after death could be something inherent to all ancients but as far as I am aware there is nothing that says it is.

Also you are sort of right when you say they don't die when their physical bodies do. As evident by Emet, Hythlodaeus and sundered like Livia, Ilberd, Minfillia and others when people in FF14 die they do remain themselves in the aetherial sea for a time, but evidently they don't permanently remain the same person in the aetherial sea as otherwise every character we meet would be able remember their past lives and the sundering memory loss wouldn't matter after a generation. Not to mention their are real world religions where when you die you still exist as yourself in the afterlife so that doesn't mean existence after death means you never died.

[Lurina]

If an ancient has to become a primal, merge themselves with a mythic beast or cheat death by going to an in-between world when they should be returning to the aetherial sea to become immortal it doesn't mean they are naturally immortal. If a modern human found a way to become immortal that only affected themselves you wouldn't go around saying that all modern humans are immortal would you? To be fair whatever it is that the unsundered did to return after death could be something inherent to all ancients but as far as I am aware there is nothing that says it is.

Also you are sort of right when you say they don't die when their physical bodies do. As evident by Emet, Hythlodaeus and sundered like Livia, Ilberd, Minfillia and others when people in FF14 die they do remain themselves in the aetherial sea for a time, but evidently they don't permanently remain the same person in the aetherial sea as otherwise every character we meet would be able remember their past lives and the sundering memory loss wouldn't matter after a generation. Not to mention their are real world religions where when you die you still exist as yourself in the afterlife so that doesn't mean existence after death means you never died.

In a scene in The Ocular in Shadowbringers, Emet outright says that his own longevity is nothing unique, and there was a time when everyone could expect to be able to live as long.

"I do not claim we Ascians are special. That is another misconception. In the beginning, everyone - everyone lived nigh for eternity. Such was the natural order of things."

Stock villain speech aside, it's a bit of a strange boast to be making in the company of a couple of other supposed 'gods'. If the Ancients are all immortal to begin with, then why is that so special? Why does that quality make him, specifically, a 'demi-god'?

The Ancients are mortal insofar as they are all flesh and blood - they might have essentially perpetual longevity, but as we see in Akadaemia Anyder, being mauled by a fish monster will still do them in. Implicitly, Hesperos is talking about ascending to a state where his power is such that he can no longer be killed.

[Kordarion]

In a scene in The Ocular in Shadowbringers, Emet outright says that his own longevity is nothing special, and there was a time when everyone could expect to be able to live as long.

"I do not claim we Ascians are special. That is another misconception. In the beginning, everyone - everyone lived nigh for eternity. Such was the natural order of things."

My point with the Emet-Selch part isn't that he lives for a long time but his ability to cheat death, where he dies but doesn't return to the life stream, doesn't appear to be a normal ancient thing. Also I take a lot of what Emet-Selch says in Shb with a pinch of salt considering the fact that he is actively trying to convince you to switch sides to join him, plus both Shb and Endwalker show Emet has a flare for the dramatic so he's possibly overstating here considering he also said they lived for an age. Two vastly different periods of time.

Edit: Not to say I think the ancients aren't long lived, that's for certain, just that Emet-Selch seems like he likes to be hyperbolic on occasion. Also in the traditional sense that's exactly, what immortality means, that you cannot be killed at all. The Oxford dictionary evens describes it as such,

the state of living or lasting forever