The problem isn't focusing on the plot - what people are complaining about is that the content people actually care about is completely shafted and sidelined because whenever Hyao gets some random idea that only he or Brim care about, then he has to put it into the game. Even when it comes at the active detriment of all other content because it completely sucks away their writing focus from everything else.
I'd be entirely fine with them expanding the world and adding new shit to fuck/be fucked by if they also remembered to add more scenes of fucking/being fucked by the actual characters they've already added. But they don't do that, and instead laser-focus on "new thing" until they get bored of "new thing" and then go do something else until they get bored of that too.
It actually has everything to do with the plot. Or rather, narrative writing.
That's why so many projects died cuz of the burntout of "adding new thing > get bored so move onto the next new thing > repeat", just like you said - The dev themselves get bored of the whole eventually.
Even gacha that's solely exist to sell "new things", still need passable plots to get people attached.
If you just put random fucks in a empty "sandbox", it's still empty.
A good sandbox game provides all the tools you need to make fun contents yourself.
But 99% sandbox porn games don't hv that, you just doing some menial busy clicks for repeated fuck scenes.
The only "fun" ones I found are actually porn mods for the likes of Fallout, The Elder Scroll (Skyrim), The Sims (4), even stardew valley. They are fun games first, then ppl making porn mods.
The charm of Lustful Desire is these amazing characters (art, writing, etc) and hot sex you encounter on your adventure - this itself is narrated, the "Gameplay" part sucks big ass.
But if they go back to this trend that just adding new random NPC that no one has attachment to, without actually progressing any of them so that ppl care, without ever bring closure to anything, then it's very worrisome.
The game stagnated for years because of that.