---SPOILER WARNING---
It's so interesting that in the "happy ending" when you check the menu, you see that Lera's portrait is missing. I don't know if it was intentional, but to me it seemed to symbolize the dissolution of her own identity into the fleshy hive mind necessary to achieve that kind of artificial happiness.
This game was beautifully crafted, so much so I can imagine a movie version of Flesh, Blood, & Concrete with real life actors lol. I think regardless of background, a lot of people can relate to the idea of wanting to do anything to get away from pain, which makes Nika's arguments persuasive on the surface. But the sad reality is that the only way to have pleasurable happy experiences is to open yourself up to the possibility of displeasure and discomfort, at least sometimes.
Also on a side note, it would have been so easy to make Nika the typical deceitful creepy child horror archetype. But the fact that Nika wholeheartedly believes the things she's saying because that's been her reality for decades and that's the way she's coped adds so much to the themes. The relationship between Lera and Nika is honestly one of the strongest elements of the game.