[timr]

Maybe you're right that it's all just hard-headed stubbornness from fluent people. But I (and all other learners of Japanese) started by reading hiragana, and only later did we move to mixed script -- this is by necessity. Yet even when you factor in the difficulty of learning to read Kanji, reading Kanji is vastly easier than reading kana, even as a beginner.

Would spaces magically solve this problem? I guess it would solve some things -- you'd no longer have to guess where to terminate the prefix search, and I think you're right about word shape -- but it would definitely not provide the additional semantic context you get from having the high-bit-density characters in the mix. This makes reading faster.

I suspect that one could make a kana-only writing system that would be functional enough, but it would still be slower to read than mixed script. Also, the Korean comparison isn't exactly valid -- Korean has more sounds than Japanese. It seems minor, but Japanese has a ton of homonyms because of the tiny phoneme. Expanding that, even slightly, would be a benefit to reading.

[coffeeling]

Korean faces the exact same problems as Japanese, though - the language structure is similar, they have a ton of Chinese loans, and have in general gone through a largely identical history of writing development. They have somewhat fewer homonyms than Japanese, sure, but they still have tons from Chinese loans (hell, "coffee" and "nosebleed" sound the same, as do "blood" and "rain" in many cases).

It's somewhat hard to believe that Japanese sits in some magic spot where a phonetic script wouldn't work just fine when Korean does it fine, and on the Sinitic side people write books in pinyin, Vietnamese is phonetic, and the Dungan people write their 3-tone Mandarin dialect with cyrillic alphabet without even notating tones.

> Maybe you're right that it's all just hard-headed stubbornness from fluent people.

It's not just hard-headed stubbornness - reading kana really is more difficult to proficient readers of today's Japanese, and change is work.