I wrote a long reply before but lost it. But I am sure demand for cute young-looking girls is timeless. And if this is true for anywhere it is true for Japan! It seems more likely big changes are due industry factors/politics. In short, I have read here about legal changes (like girls now allowed to have their scenes removed from sale), changes in how studios are run, shifting power dynamics between girls & studios, and of course how scenes are distributed and consumed is not the same as it was 15 years ago.
I don't know enough to say what are the big factors. But with the example of bukkake, there are still tons of girls in the west who are happy to do the most depraved things for reasonable payment. Here again I don't think girls getting drenched in cum would remain popular in the west while people lost interest in Japan, of all places. Also would any of these genres go straight from ubiquitous to silent? I'd think you would see a tapering off over years if there were just changes in audience tastes.
Basically, you have to consider the source and whether it is seems reasonable:
- Girls not available (either less pressure or less incentive to do work, changing culture (alternatives to JAV, societal attitudes), ?)
- Studios don't want (less profitable because of (demand (nobody wants to see young girls or bukkake/gokkun anymore), girls pulling their scenes too soon, market saturation), legit management doing more "above board" type of scenes instead of mafia types coercing more girls to do more things, ?)
- Studios want and girls available but some other obstacles (haven't adapted to new patterns of distribution and viewing, some new legal risks, trouble recruiting (girls not comfortable with studio or male actors, outreach methods behind the times, ?), directors not available, ineptitude, ?)
Are there no industry press in JAV, or insiders who have given/can give interviews about this kind of thing? I feel like that would give the best insight into bigger trends.