I think the issue with EyeSight is that Apple wanted to create the illusion that your eyes were positioned where your face is (and not where the front glass is)

I think the issue with EyeSight is that Apple wanted to create the illusion that your eyes were positioned where your face is (and not where the front glass is), and to do that required a lot of compromises to image quality. Replacing EyeSight with an Animoji or whatever wouldn't fix that fundamental issue; you'd just have a dark and blurry Animoji instead.