Universe Mechanics Conflict Often [Spoilers]
I tend to like the story, but the cosmos of FF14 seems wildly contradictory and it seems to get worse with every expansion.
I was down with some of the messages and thought processes in Endwalker. The Ea being depressed about entropy being a conclusion to existence is a dysmal prospect to immortality.
Only, the setting seems to have decided entropy isn't a thing at random. Like, I don't know, Valigarmanda creating eternal fires that don't even need fuel to burn. Endless energy? Or how about the Sin Eaters that the lorebook says don't need energy to function. Or how about the fact the story introduces those two voidsent who somehow spared their hunger by just feeding on each other?
One of the biggest reasons the primals were being hunted is they'd feed on all the aether on the star (Though the evidence of this actually being a worry is spotty) when you have infinite aether fires, how is that a threat?
There's just countless situations were energy is undiminishable is presented, and usually adds no story value to have it. Why have eternal fires for Valigarmanda? To show how powerful it is? A crater that bored through to underground lava vents would have show the same power without the need of perpetual energy.
Bit late to this party, but here we are.
I'm a bit late to this, but hopefully I can help elucidate upon some of these issues.
Let's start with the easy one. The Primals thing.
You're right that from all appearances, a Primal feeding on aether when aether doesn't seem to run out seems inconsistent, but there's two main factors to keep in mind. Firstly, a LOT of the "Primals will sap the land dry." mentality stems from in universe beliefs, at least some of which on this topic are pushed by the Garleans who are explicitely using the Burn as evidence, which is later explained to NOT be a result of aether being drained at all but rather it being stopped from flowing in, and the story about it being just propoganda to justify the conquests. The real issue from Primals, as demonstrated with the Eden raid series, is that they Aspect or Polarize aether to their associated aspects/polarity, which can lead to major issues in an environment. Remember, it was Eden forming and proceeding to do just that that led to everything else that happened in The First.
The next one is the Valigarmanda fires. This is actually a REALLY interesting case, because it ties into something that doesn't get talked about nearly enough, which is the impact that Regional faith has on the environment! Those fires are eternal because The local people believe that they are eternal. What I mean by that is that those fires, like Valigarmanda and other Tural Vidraal, or like Auspices, or Kami, or Tsukimogami, or Primals like the Twelve, are being influenced, and to an extent defined by, the collective regional faith of a whole region. in this case, the New World. Getting into the fine details on this one would necessitate an entire document on the matter (which is why i've been working on one.) so I won't go into fine detail right here, but if Valigarmanda were to be fully forgotten, and folks forgot the fires are supposed to be eternal, they'd likely go out. They're essentially enchantments that burn perpetually, rather than being "natural" fire.
As for the Voidsent and Sin eaters thing, you're actually right that there seems to be no loss of energy between them. their hunger is a result of an instinctive response to the aetherial imbalance they're formed by, but otherwise they don't NEED more aether to continue existing exactly, but they ARE noted to diminish in power over time if they don't feed SOMEWHERE. The fact they can reform after death given enough time is likely due to a reduction in the aetherial loss in the interim.
The rules aren't as inconsistent as you might think, they're just fed to us from a LOT of in-universe perspectives and views, and so are sprinkled all over hell and back rather than being any one collective place that'd be easy to parse. it's pain for those who don't spend hours researching it all, but for those of us who do, there's a lot more consistency than it looks at first glance. it's just SO NOT WELL LAID OUT. T.T
Also, on the Ninth thing, there's a LOT of uncertainty about the Ninth and the entire timeline surrounding it, even for those of us well versed in all the random bull that lore has. the Time dilation shenanigans of the Shards make it REALLY hard to say anything conclusive about the shards.