I’m not going to discuss about anyone’s political bias here. I’m just interested in the average reading/writing ability of the average adults in today’s Japan. And I’m afraid your assumption seems to be extremely underrepresented.
Firstly, the rate of enrollment in compulsory schooling is very high in Japan. It is a widely-known fact even if you don’t trust the officially announced figure at all. Pupils are required to learn 1006 kanji in the first 6 years, and 939 in another 3 years.
Secondly, our society does not assume a person who didn’t go to high school an ‘average person’ although people realize there are a lot of able people who don’t finish high school. It is education-conscious society for good or ill. On the “official data”, the rate of advancement to high schools is approximately 96%.
Thirdly, reading/writing ability is considered to be essential in our society. In fact, many employment-offers require handwrote resume, even for some minor part-time jobs such as dish washing. It might be the case in any country, however, our common sense does not accept people who can’t even read the shop’s manual.
And lastly, in terms of the “common sense”, there exists a rough guide which evaluates people’s ability by the amount of kanji they know. Here is a criterion for the Japanese Kanji Aptitude Test. http://www.kanken.or.jp/teido/
Level 10: Tests 80 kanji (1st grade of elementary school)
Level 9: Tests 240 kanji (2nd grade of elementary school)
Level 8: Tests 440 kanji (3rd grade of elementary school)
Level 7: Tests 640 kanji (4th grade of elementary school)
Level 6: Tests 825 kanji (5th grade of elementary school)
Level 5: Tests 1006 kanji (6th grade of elementary school)
Level 4: Tests 1322 kanji (junior high school level)
Level 3: Tests 1608 kanji (junior high school graduate)
Level 2: Tests 1945 kanji (high school graduate)
Level pre-1: Tests 3000
Level 1: Tests 6000
This list seems to be fairly representing the common perspective in Japanese society although it doesn’t prove anything about the actual ability of the people. However, it would be one of the evidences of the fact that an adult who knows only 800 kanji will be held in contempt in our society.
Note that I’m not talking about political issues such as necessity of educational reform. I don’t have any objection against the opinion that a deterioration of academic ability is recognizable today. But, as I said before, claiming “average adults know only 800 kanji” is nothing but a dismal warning if I refrain from using the word propaganda. It works only in the political context.