@Nihilist1 Wrong. One Piece is an adventure. It's just that the story is not about the treasure that Gol D Rodger left behind. The One Piece itself is a MacGuffin. But each arc actually matters to the over all plot of the story. They all connect and actually matter to the growth of the chapters. Each arc actually matters to the growth of the characters and how they reach the One Piece and the story's end.
One Piece is more about the adventure and how the characters interact with it.
For Nisekoi, the drama was more of a MacGuffin as well. It was just an excuse to bring all the characters together. To have them unite. It was over one hundred chapters of fluff. Not every single chapter for Nisekoi actually connected with the plot. And not every character added to the harem would need to matter to the MagGuffin.
Think about it. For the love triangle, even if you removed the other girls with the promise and the imouto and took away a huge chunk of the characters, you could still have the love triangle plot happen. They weren't necessary for the Love Triangle plot or for the promise. Because in the end, Nisekoi wasn't actually about the love triangle. That was just an underlying part to connect all of the stories. Like the infinity stones in the MCU.
So if you think about it another way, the over all story was never about the love triangle. It was just something to be used to start and end the series.
And we are talking about this because I'm saying that Nisekoi is different. And Nisekoi never went down the gutter like you claim. You just misunderstood what it was and expected something different.
I felt it was pretty obvious from the start that Nisekoi was going to be about the slice of life and the characters more than the series underlying plot. I mean, the so mystery only got real focus in the very beginning and the very end, with every other moment instead being used to just remind you that it existed rather than being there to actually explore it.
Another series that did something like this was the Magic Tree House books. Each book was technically it's own story and there was technically an underlying mystery, but it was never about that.
And most harem manga are this way. They use a MacGuffin to give a set for the series, but in the end, it's not the real focus for the series. The characters are. The MacGuffin is just used to bring people together and to give the series something to end with. Once the MacGuffin is solved, the story would end. That's it's real purpose.
So you viewed Nisekoi as a failure because you misunderstood it's very nature. The love triangle was like the setting for the series rather than it's point.