Japanese already has two syllabaries - katakana and hiragana - so if anything replaced kanji it would be these. However Kanji makes reading Japanese a lot easier (if you know the characters), as it helps with:

* parsing sentences - kanji make word barriers clear

* disambiguating similar sounding words - Japanese have many words that sound the same, but mean different things - these words have different kanji

* reducing text size - one kanji character corresponds roughly to two katakana or hiragana characters (for example, this means Japanese tweeters can say much more in the same number of characters http://twitter.com/kharaguchi/status/22214712818)

So for Japanese, using kanji appropriately is more efficient for the reader than the syllabaries, and certainly romanized Japanese.