I don't know anything about Korean. It's a different language, and I'd imagine that some of the things I'm saying here don't apply to it, but that's a guess.

For thought experiments, you're assuming your consequent - obviously a society raised without kanji would get by without kanji, though I think the language would have to change somewhat. But for non-hypothetical people that exist now, I think the things you've been saying about kanji not making it easier to read Japanese just aren't true.

Japanese learning materials for Japanese 5 and younger are hiragana only, and I remember finding it annoying.