From a hypothetical perspective I'll concede that it's not impossible to be effective in Japanese without kanji (after all, Korean is very similar in a number of critical ways here, and has been functioning largely without Chinese characters for 60 years—and North Korea has completely abolished their use), however there are many practical benefits to be gained from keeping them.
That said, if you remove the kanji, it actually makes (written) Japanese much harder in several ways. The most important of these is that it makes it more difficult to identify homophonic words.
For example, which meaning is the writer referring to when they use the following words?
- sake: salmon (鮭) or alcohol (酒)?
- hashi: bridge (橋), chopsticks (箸), or end (端)?
- kami: deity (神), paper (紙), or hair (髪)?
- kaeru: to change (換える, 変える, 替える, 代える), to go home (帰る), or just a frog (蛙)?
These are just a small sampling among many others possible candidates. These don't even list out all possible homophonic examples for the ones I've called out.
Further, for experienced readers of Japanese, it becomes much harder to skim through things because the visual cues have been removed. Kanji aren't just there to make people's lives difficult.
Another area in which Kanji are useful is that they provide a built-in system of roots, prefixes, and suffixes to use when learning the language. Now, it's true that other languages have this as well (most European languages get theirs from Greek and Latin, for example), however kanji once again provide a distinct benefit here. By having a defined visual representation, kanji provide a specific, unambiguous clue as to the meaning of the word in which they are being used.
- 歯医者 = 歯 tooth, 医 medicine, 者 person = tooth + doctor = dentist
- 文学 = 文 writing, 学 learning = literature
Further, this information is taught as part of learning the language—I've met several adults who speak English natively that were not aware of the concept of Greek and Latin roots. This awareness allows not just for the deciphering of new words you come across for the first time, but also enables someone to make educated guesses when they need access to a word they haven't previously seen or used.
For example (from my own experience):
- If you're trying to say something happened at the same time as something else, you could combine the knowledge that 同 means "same" and 時 means time to get 同時【どうじ】.
In short, when approached with the right attitude and understanding, Kanji make Japanese a far simpler language than many others, for the following reasons:
- Japanese grammar is far simpler than most languages in critical commonly-used areas such as noun, verb, and adjective usage (no need for separate conjugations, numerical agreement, gender agreement, etc.)
- Kanji provide a highly-visible method of guessing the meanings (and in compounds, many times the readings as well) of words that you have never come across before
- In a similar manner, kanji provide a means of visualizing and constructing new words to fill needs much more fluidly than in most other languages