It's the readers' problem if their expectations are not in line with what they get. I agree, the arc is plain stupid but that means nothing but some sort of a creative block on the mangaka's side.
Actually that's the authors problem. If, as a writer, you make a promise (deliberately or unknowingly) to your reader that " X is the plot and thus Y is the inevitable outcome of that plot" and you violate that promise, your reader has every right to be upset. Even tone consistency is part of the promises an author makes to their reader and a good storyteller does not betray their readers by "suddenly shifting to a different story that in no way matches the tone you started with. For example, if you start out with a dark and brooding tone, you don’t want to pivot and reveal that your novel is actually a lighthearted romantic comedy."
Oshi ga Budokan Ittekuretara Shinu is a great example of this - promised one thing (yuri) and delivered a completely different story (we can't ever be together - not even as friends). That one didn't just not deliver the story they were promising, it actively took efforts to tease the story they promised over the one they delivered.
This whole NTR arc was not what most readers were signing up for when they picked this series up. In fact, I would argue this series popularity is partly in thanks to the fact that it doesn't drown itself in unnecessary villianry, sorrow, and angst that ruin so many other potentially great yuri series.
Bottom line - if your promising your readers cute fluffy yuri romance, you keep that dark **** out.
"There's nothing like a reader scorned, and we've all seen the Amazon reviews to prove it"
