In theory, Japanese can be written entirely in romaji. In a short sentence, most Japanese speakers can usually follow the sounds and understand what it means. Writing Japanese in the Latin alphabet does not make it stop being Japanese.
The problem becomes much more obvious in longer text. Japanese has many words that are pronounced the same but mean different things. If all you see is a sound such as kōshō, it may not be immediately clear which word is intended. With kanji, the difference can often be recognized at a glance. In romaji, the reader has to rely much more heavily on the surrounding context.
The visual structure of Japanese also matters. Kanji, hiragana, and katakana appear together on the page, and that mixture helps readers see where words and phrases begin and end. When everything is written in romaji, those visual boundaries become less obvious. The text is still readable, but reading it becomes slower and more tiring.
Kanji also does more than distinguish words with the same pronunciation. Japanese readers often take in meaning and imagery from the characters themselves. A compound can suggest something through the meanings of the individual kanji that form it, and the choice of characters can add shades of meaning beyond the sound alone. Romaji preserves the pronunciation, but it removes this layer of information.
Romaji is still useful in many situations. It helps visitors read station names and personal names, and it can be useful for people who are just beginning to learn Japanese. But replacing all the writing in a newspaper or a book with romaji would make the text much harder to process.
For a Japanese reader, an entirely romanized passage is therefore more likely to look like a learning aid or a form of support for non-Japanese readers than ordinary Japanese writing. Japanese can certainly be written in romaji, but removing kanji, hiragana, and katakana also removes many of the visual and semantic clues that make the language quick and natural to read.
