Honestly I see nothing that points to the crystal being sentient, not any more than any rock at all, and we certainly never tried to go see what was the centre of the Swamp of Death either, to converse with it or anything. The ES feedback was "put into words" but in the same way it was with the rest of the earth and stone Hazō tried the technique with - and that feedback was not even from the crystal, but from the ground, away from it. Our best guess from that IMO should be that just like with chakra adhesion, the presence of high levels of nature/natural(?) chakra here, concentrated in the crystal much like a chakra battery, automatically reject others types coming in.
Hell, it's not even just that - the only confirmed case of chakra creating life out of no life is chakra golems, which generate in the precise conditions seen there. Even them we don't know to be sentient, considering the difficulty of surviving the encounter. I am of course not against keeping an open mind and looking for signs of sentience, but it's very much not the zero hypothesis and it confuses me that we keep it as a starting point.
And the thing is, high concentrations of chakra makes dangerous wildlife no matter what, apparently. We can't stop that, and Uplift was supposed to try and solve this war of attrition humanity is losing. Not preserve every single theoretical life - which is infeasible. Best case scenario for the crystal if it's sentient and can be reasoned with would be to convince it to burrow itself so deep underground even a chakra golem generating near it couldn't come out. And then hope we didn't need that material it's made of to practise lithosealing because if the Dragons ruin the Seventh Path and gain the ability to travel to ours they'll kill it too.
It's not the earthshaping "voice" that I'm thinking of when I say it might be sentient. When I originally arrived at this position I had honestly almost forgotten that part of the exchange.
No, what has me unwilling to immediately lump it into the "beast" bucket is that it appears to be an ex-nihilo intelligence with cognitive faculties at least sufficient enough to control or create the vine golems we saw, which fought in real time with considerable acumen. The ex-nihilo part, though, means that we don't have a reference frame for what the brain of an entity like this is supposed to look like. If it obtained intelligence all the way up to beast level, all on its own external to any and all biological evolution, can we be sure it stopped there? I mean, certainly it
could, but the crux is that we have no way to be sure without further investigation.
And I must stress this: I do not think the cave is sentient, I think the cave
might be sentient. I do not think we can reasonably rule out the idea that this ex-nihilo intelligence has personhood and an inner life, even if it's fundamentally alien to us. In the non-negligible chance that it is sentient, we have an obligation to treat it like a sentient being and try to save it even if we ultimately fail to find an outcome where it can be allowed to live. I don't like not knowing the answer here, because the actions we should take are very different depending on the outcome, so I very much want to figure out the answer when we next go back to it. I'm not committed to the outcome, I just want to
know the outcome instead of guessing that this esoteric and unique being happens to follow the rules we're accustomed to.
With respect to chakra golems, I... don't
think the causality is in that direction? The crystal cave seems to me to be a byproduct of heightened chakra levels in the area, much like a chakra golem would be, but not the
source of it. Destroying the crystal would stop the crystal from living in the cave system and killing people, but I don't think it would do anything to bring the chakra levels in the area under control. I can understand being worried that the crystal cave is a proto-golem or something, but that's again something that we can't say yes or no about until we know more about it.
(as a side-note, you make a good point that the crystal itself might not even be important to the entity at all. It makes sense for the fancy chakra-dense crystal at the heart of the cave to be its brain or something, but that's an assumption on our part that might just be flat-out wrong. Which, again, makes me want to learn more about it so we can figure out how it ticks and what it thinks)
But I mean, it still feels weird to theorise about the need to save the crystal even if it's sentient given the danger and all when the cast is not even vegetarian. There's saveable sentience right there
Addressing this point as a specific thing, sorry if I made any miscommunication by saying 'sentient' instead of 'sapient'. I'm aware that 'sentient' as it's formally defined includes all sorts of non-sapient animals, but I was pointing at the colloquial meaning that's equivalent to 'sapient', i.e. "the being is self-aware and has personhood" or something along those lines.
I do think that going vegetarian would decrease the number of sentient (defined formally) beings our cast kills, but I don't think it would decrease the number of sapient beings our cast kills. I am unsure whether or not the crystal cave entity is
sapient specifically, and if it does have self-awareness and personhood I would consider us as having certain moral obligations that would not exist if it was merely sentient (defined formally). That it is sentient (defined formally) is virtually certain, given the vine golems it attacked us with.