Adding whitespace is a pretty simple solution. Heck if you really, really absolutely needed to resolve tonal ambiguity in kana you could add something to Kana to do that. That'd enhance the readability even further since, it's basically impossible for foreigners to learn correct intonation in Japanese unless they explicitly study it and that's on top of memorizing all that Kanji, but it would become explicit. I can recall exactly once in the last 10 years having a conversation where the there was ambiguity between two homonyms and someone asked a clarifying question to resolve it. The vast majority of the time it's just clear from context.

So.. I would say even that ambiguity isn't something people would actually have much a problem with.

That is what whitespaces are for, which Korean also uses.

Sometimes furigana can be used artistically,sure, but that's the exception to the rule and it's by and large a reading aide in the vast majority of cases, and the inclusion of it in novels aimed at adults indicates that without it the author expects a certain percentage of readers may struggle with how to read the Kanji otherwise.

Why does this tool in the language need to exist? The answer cannot be because Kanji make things easier to read, else you wouldn't need tools to help you read Kanji you at times otherwise wouldn't be able to.

If you come across a word you don't know as either a native speaker of English or Korean, you can at least sound it out, which ups the probability you can connect it with a word you've heard before, otherwise since you know how to type it out it's trivial to look it up in a dictionary. If you come across a word you don't know in Japanese as a native speaker and there's no furigana it's a guessing game. The meaning is slightly more obvious to you, so you might be able to guess, but if you can't guess and you care to know and the word is in print then looking it up becomes a bit more of a pain.

Korean didn't completely lose the mutual intelligibility aspect entirely since the underlying pronunciation of the words still remains and can be used to correctly guess the word in a lot of cases. Like 시간 and 時間 as an example, but there's many, many words I've been able to guess in Korean based off knowing Japanese. I was able to score 50% on TOPIK II reading exam after only having studied Korean for 4 months in large part because of this.

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.