Problematic notion about Zone 5 & 6 story bits.

[LilimoLimomo]

That said I never thought that characters from ARR to Endwalker would hold such strong beliefs of what you can and can't do with a soul.

Oh, 100%. It's one of numerous ways that Dawntrail's narrative is flawed. Because since ARR, we've been doing all manner of occult madness with souls, such as injecting thousands of them into our relic weapons. And suddenly someone has a problem with how souls are being treated? Yeah, it sounds hypocritical, it sounds ethnocentric.

To be clear, I don't think Living Memory needs to be shut down because it's bad to use souls or whatever*; it's entirely about them needing to harvest an endless amount of energy from other living creatures to sustain their own existence. Like the ARR Primals, they gotta go.

*though since ARR, I have very much raised an eyebrow that everyone is cool with harvesting souls like that...

Had Sphene not presented her ultimatum and started the interdimensional fusion, we would have 100% tried other solutions to keep these memories "alive".

I don't think we would have; at some point in the story in Living Memory, one or more of the Scions state that from the very beginning, Living Memory was doomed to failure because the system's design is unsustainable. Quite simply, Living Memory is computer storage that needs to be able to infinitely grow (to hold more data, more memories), and the bigger it gets the more resources it will require to keep the lights on. But nothing can infinitely grow; it's not just a fact about nature, it's a fact about reality. Resources are finite, and thus infinite growth is always doomed to failure; infinite growth cannot be sustained. I think it's a bit poetic that in reality, the only natural phenomenon that attempts to infinitely grow is cancer.

The writers were correct to have the Scions point this out...though they were sloppy in having the initial engineers of Living Memory fail to realize this incredibly obvious flaw in their system. Any engineering team worth their salt would have noticed that problem well before the project was finalized, and any population that truly loved their dead wouldn't have generated simulacra of them that would need to choose between their own existence and destroying all life throughout the universe. It's a glaring plot hole.

I could argue that the Aetherial Sea is just a different version of Living Memory, they just operate differently with different sets of beliefs.

At least according to the lore we're given, it's not; the Aetherial Sea breaks down things like souls and memories so that they can form new life, and that life will eventually be broken down itself, and so on and so on. It's the circle of life. But Living Memory just keeps the souls and memories so that they don't return, and thus new life can't be created from them.

And again I have to emphasize, according to the lore we're given, the simulacra in Living Memory are just computer simulations. It's not an afterlife, it's a room full of ChatGPT bots that have each been fed memories related to a specific dead person so that they can impersonate them.

[Kirutsuki]

I don't think we would have; at some point in the story in Living Memory, one or more of the Scions state that from the very beginning, Living Memory was doomed to failure because the system's design is unsustainable. Quite simply, Living Memory is computer storage that needs to be able to infinitely grow (to hold more data, more memories), and the bigger it gets the more resources it will require to keep the lights on. But nothing can infinitely grow; it's not just a fact about nature, it's a fact about reality. Resources are finite, and thus infinite growth is always doomed to failure; infinite growth cannot be sustained. I think it's a bit poetic that in reality, the only natural phenomenon that attempts to infinitely grow is cancer.

Yea I agree with this and would like to add to it that the flaw was Sphene herself and her love for her people.
Since she wanted to preserve her people even after death no matter what or who they were, it means that she couldn't let go of them after they have become Endless.
Which resulted in the Living Memory's capacity to fill up.

But see, that is never directly stated to be that's the case, you have to do a bit of mental gymnastics to get there,
which is to say that the writing is falling short on this area in that regard.

They do mention at points that the Sphene we see is not exactly the same as the real Queen Sphene of Alexandria, because she was made from the memory
and because of that her need for preservation of souls became absolute. So the Living Memory really only failed BECAUSE of the version of Sphene that was there.
Even if she did still harbor some emotions that she stripped entirely at the end it wasn't enough for her to be able to let go of her own people.

The thing is Living Memory doesn't need to keep growing to perpetuity. If there was a version of Sphene that understood that SOME people want to actually die, and some can
find happiness there when they fail in life it wouldn't necessarily just keep on growing. Sure after billions of years it would probably be filled with the most selfish people you could imagine who only think about staying there as long as possible.
But properly managing that from the get go of who get the regulators and who don't would even fix that small issue.

The only problem is the source of the power, but with less people inhabiting Living Memory there is no real reason to invade other reflections, since the Electrope can manage quite a bit on it's own I wager.