In the real world nobody is masochistic enough to not adopt spaces if writing without kanji.
Old Japanese videogames couldn't use kanji due to technical limitations. They wrote in all kana but used spaces to make the text easier to read.
Modern Japanese children's books and eg. even Pokemon games still? Same thing, kana and spaces.
When Korean transitioned away from Japanese-style mixed script to purely alphabetic writing, what did they do? They adopted spacing.
The only time "but Japanese doesn't have spaces" comes up, ever, is when people argue against the removal of kanji. It's not a realistic scenario, in light of very recent history and current practice.
In a world where hiragana and katakana are adopted to eliminate characters, it's not that hard to imagine also adopting spacing. Hangul for Korean did not originally have spaces in the language.
