It is generally regarded that 1980 was the beginning of the end for Nikkatsu and the Roman-Porno series. The proliferation of VHS video, and some staggeringly stupid management decisions would lead to the studio's closure some eight years later.
Closer to home, there was the retirement of Tani Naomi. Now Naomi was the symbol, the icon of Nikkatsu SM porn; what the Bell UH-1 was to the Vietnam War or the Saturn-5 to the Space Age, or the AK-47 to Revolutionary Communism. This was an age when exploitation cinema had the most astonishing, exotically beautiful women: Soledad Miranda, Edwige Fenech -- and Naomi was Queen of them all. How do you replace an icon like that?