Take your own advice, and actually read what's being written. Everyone who criticizes the last zone already fully understands this fact about the endless. The issue is not about whether or not erasing them was the solution, the issue is that the writer thought they could get away with glazing over the gravity of what's being done with a "lighthearted" tone. The endless are real or not real at the whims of whatever tone the writer wanted for a given scene.
Why didn't we spend longer on the question of whether they are truly living? Why did none of them want to stop us, especially the ones who only found happiness after death? Why was there
no conflict whatsoever besides remarks of "this is kind of sad huh, oh well!" from our companions, relegated to optional dialogue as passive quest npcs. Why were we deliberately shown that the endless are thinking, emotional beings, capable of learning new information and growing, showing again and again that they are functional indistinguishable from living people?
Why were the Endless, who all came from a society that was practically
built around the fear of death, suddenly just fine with being relegated to complete oblivion when we come to kill them in the afterlife they were promised? The answer is because the author wanted to have his cake and eat it too. "Oh [player], aren't you so sad because you have to erase these people and make Erenville and Krile have tearful goodbyes with their parents ? Don't worry, they aren't really that broken up about it, because I didn't want you to feel TOO bad

"
Yes, it was necessary to erase the endless, because the author chose to write them that way. And yes, what we did was horrendous despite its apparent necessity. The tone and proceedings of Living Memory should have been informed by this, and instead it was a happy-go-lucky romp through disneyland, putting on plays and taking history quizzes, having a great time committing genocide. Oh and Erenville seems depressed, better
completely ignore him and his suffering until the end of the zone so he can have his big moment with his mom.
The writing of the Endless is atrocious no matter what way you look at it. That is what people take issue with. You think we're struggling to grasp a "simple concept", but that's because the "simple" surface reading is the only thing you're grasping at all.