I have to say, that this:
Wrong question. "Why invest 10 month of development time in a secondary trait of the product" - and likely more than 10 month, since there will be bugs, and you will need to waste even more time fixing that.
Seems to be answered, by this:
It is a plot-intensive almost-VN with erotic subtext.
Your own answer suggesting that it is, perhaps, not a secondary trait of the product, after all & despite the fact that no-one has ever claimed that it is a secondary trait, why do you believe that the story is a secondary trait & to what?
This post suggests that, because you got a story driven game, instead of the sex scene driven game that you imagined it should be, you're not happy.
Sorry to say, but that's simply entitlement.
Honestly, being a DIK is getting updated three times faster than that, despite throwing entire plot arcs with gigabytes and hours worth of content.
This seems to be another answer to your own question, provided by you.
If that games does what you want from a game, then maybe just go and enjoy it, instead of complaining here that this game doesn't?
Wouldn't that be more, productive?