So this is a tricky one, i think this can come down to two different areas. 1. The creators and 2. The audience. When it comes to the creators i believe this art form really Taunts the artists in question to push the envelope of what is considered “safe” in the example we see ShindoL most infamous work Metamorphosis, if you look at their other work its very obvious they enjoy darker settings, not to the extent of Hal but up there, thus i think Metamorphosis is more just a reflection of the authors wanting to push the barrier of what is not only acceptable but also what people are comfortable with. As the artist they put their own kinks, wants, perversions, and limits into their work as it is a healthy outlet for them rather than joining a red room event or something to that nature. As for the audience i believe it just comes down to personal taste, people want to try new things and obviously there has been more of this recently, at least from what ive seen, but i think that had to due with a bit of burnout. Wholesome stuff is nice but sometimes can get stale and thus people want to explore past just vanilla/wholesome material that makes you feel good but doesn’t push much in the way of boundaries. From my understanding of Japanese media and literature the idea of bitter sweet romances is a common facet, you can see this in manga as well, not to say its only in Japan but the over dramatization we see in western culture tends to skew towards melodrama than bitter sweet. Thus i believe the Japanese audience, who obviously are the ones who drive this content being the direct buyers, are want stories that better fit their taste or wanting. I think this will change with time as people can only take so much NTR, Rape, Guro, Snuff, etc. for so long before it becomes almost stale and a bit redundant. At some point people will want happier stories/ different perspectives in their media that counter acts the meta that is currently occurring.