Sometimes you get complacent when you become the big dog. No longer hungry. No longer innovating. Bitmain is raking in so much they just stopped innovating. With things the way they are, they might ease out of their slumber and try to cobble something together. Who knows.
IMO they are just focusing on diversifying their business portfolio. They are opening offices in San Francisco and Sao paulo, and have been actively investing in other businesses, not to mention their AI chips. In 2019 they will be traded on the Hong Kong stock exchange. They have made a lot of money on mining equipment but that is just peanuts compared to other ventures. Its kind of like Nvidia, we can complain about no new GPUs, but their main income is coming from AI and HPC. not geforce graphics cards.
And lets just say Bitmain could magically double the efficiency of their S9, which would be huge, it would still only be making a $1 a day. With current prices I don't think asic miners are flying off the shelves.