Try have them be educated in a kana only system and then have them try read it. They'd probably do just fine. You'd expect anything you've spend a decade doing to be easier than the thing you've spent much less time doing.
かんちょうが かんちょうで かんちょうに かんちょう された。 Is probably a sentence that definitely requires Kanji to understand the precise meaning given how many homophones かんちょう has, but it's a toy example.
> Try have them be educated in a kana only system and then have them try read it.
I can't, because there are no native speakers who learned that way, as I'm sure you know :D
But there are many learners like that, and my experience in Japan is that anyone who doesn't learn kanji has a very low ceiling on their vocabulary, even if they use the language daily for decades.
Because the reality is that it's hard to memorize 1K kanji, but if you do it then it's relatively easy to learn 10K+ compound words. Without kanji, to reach fluency somebody would have to memorize 5-10 completely unrelated meanings for "kouka", then 5 more for kakou, and so on for every combination of common single-kanji readings.
I mean - if you're in Japan, you surely know people who try to get by without kanji. Do you know any who've reached fluency? Like who could use and understand 5-6 different "kouka"s without any idea of the kanji they use? If your premise here is true then people like that should be the norm, since learning would be so much easier for them compared to those wasting their time on kanji?
So... how do Koreans and Korean learners do it? Not to mention other languages that used to use Kanji but dropped it?
You have to imagine the entire education system and everyone in it got changed to Kana only and Kanji was subsequently removed from modern literature from that point onward. That's the thought experiment. It was even tried successfully in places...
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.
かんちょうが かんちょうで かんちょうに かんちょう された。 Is probably a sentence that definitely requires Kanji to understand the precise meaning given how many homophones かんちょう has, but it's a toy example.