How many times does this have to be repeated?

It does not matter one bit whether or not the Endless were sentient or not, real or not, etc. Discussions like that are a distraction.
What does matter is that their existence was unsustainable, and as a result it would either eat itself alive, or someone like Sphene would harvest souls from outside of their sphere of influence as fuel for the Endless to continue existing. The debate on sentience is a red herring. The cruelty they would inflict is what causes them to become an enemy and it simply becomes two entities fighting for survival. And at this point, we are simply morally 'more alive'. They already had a life, they died, and they should not get a second chance at life if it means that another person's soul has to be forcefully harvested for it. If you can just make this technology exist without having to slurp up all the aether, go right ahead. Until then, the technology is unsustainable and immoral.

This is such a simple concept. But you get stuck on whether or not they're real or not.
Let's say we give you what you want, and yes, they ARE real, and ARE sentient, and they ARE intentionally harvesting souls for their own selfish needs. Does that make it better? Would you sacrifice yourself so they can continue existing? Would you force others to just let themselves get harvested for them?

Get over it and use some critical thinking skills. Sphene and Alexandria were not "grey" in morality. They were deep dark black, with a mask of cooperation and well-wishing that you were believing and you were betrayed but you don't quite believe it, so you feel guilty at what happened at the end of Dawntrail.