I think the issue the Japanese public has is, "why this, and why now?"
As I've mentioned before, the television and music landscape changed, mostly via social media and YouTube: blogs helped disseminate personal thoughts, but the best way to reach out to fans pre-2010 was going on TV. Considering Ayu doesn't make many appearances anymore and the fact that she hasn't released a critically-acclaimed album since possibly
Love Songs or
Love Again (which were both basically compilation albums of her previous EPs) back in 2013 means her fandom (including us abroad ones) dwindled over the years.
Japanese people still love gossip as much as the next person: why else are there still
shufukai (for the housewives and retired ladies) and tabloid publications like
Shuukan bunshun and
FRIDAY still create impact when a story breaks?
Twitter being the second most used SNS platform (after LINE) means they're still interested in expressing their opinion (or reading others').
The real issue was the format choice: why "fiction based on fact," and not a documentary or interview? Granted her relationship with Matsuura must have been tough to talk about if she felt so deeply about it, but from most of the Japanese reactions I've seen, the issues were it was too shallow to call it a "tell-all book" (i.e. the advertising wasn't right) and they really just didn't want to know more about her relationship with Matsuura. Some fans still appreciated knowing the background behind some of the songs/performances, some are defending her works and want people to separate what they may think of her as a person versus what she produced, and some wonder why they were fans in the first place after reading it, but "disgusted" is more of an extreme reaction (vs. an average one).
The other major problem was the fact the story come from her. For Ayu herself to be involved in it was what made it seem "desperate" and "cringey," but had it been an unauthorized work or story, then the reactions would have been more sympathetic.
Personally, I don't see anything inherently wrong in it: sure it's tacky and apparently the content is lacking, but it's already happened at this point. She can't recall the books, and make people forget what they read. The comparisons between her generation mates Utada and Amuro nor her publicized marriages/divorces and lack of children definitely don't help her case, but I imagine the difficulty in trying to create new music is hard to overcome when she's almost completely deaf.
In the same way the Hello! Project girls still respect Tsunku and want to work with him despite his reduced role, I'm sure there are up-and-coming artists (within Avex and their subsidiaries) who would love for Ayu to help them write their lyrics—that would probably be a more productive use of her time if she was pointed in that direction, instead of continuously trying to sell herself.