The reasons for not releasing singles is simple. CDs have become obsolete and the music industry has been failing to find a valid physical substitute and can only rely on digital sales. People use portable electronic devices to listen to music nowadays. I'm sure the sales of big stereo sound systems has dropped considerably over the years, though I haven't done research myself. People will barely buy a full cd album these days, what to say a large CD with 2 to 3 songs only. Local bands in my city are rarely releasing physical CDs nowadays, instead they've turned to live performances without physical releases. I don't even see physical CD piracy as much anymore - 8 years ago it was abusive
A lot of record stores have closed in my city. You can only find CDs in department stores that sells other different things. Even on those, CD departments have shrunk progressively and considerably over the years, and in a couple department stores that had large CD sections, nowadays I see nothing or maybe just a small shelf hidden on the back with outdated releases that weren't sold or 1 or 2 massive hit recent albums. I often only find some recent released albums through online shopping. I don't think this reality is very different in Japan, and ayumi is not the only japanese singer who really hasn't been releasing physical singles recently. Artists like Madonna would attempt world tours every 10 years, and now you see her advertising world tours yearly. Clearly, the world businness for music is very different nowadays than waht it was in 2001. If people fail to realize some of this history, it might indeed sound like avex is conspiring to keep some number one streak, however the reason for ayu not having released singles recently most likely relies on the market itself rather than avex trying to pull some crazy trick, after all holding the streak doesn't really guarantee much of a possibility of it not getting broken if they decide to release a single 2 or 5 years later.