1. That's just you pre-judging people, and assuming you know them based on a few forum posts. That's prejudiced. People don't like what they don't like, and have every right to voice their opinions, just like you do.
2. Making the route spilt far into the game is an absolutely terrible idea, and every game that has done it has received MASSIVE backlash. Why? because when you draw a certain audience, in this case harem fans, and then add something like ntr, the direct antithesis, way down the line then you've gone against what your core audience wants. You either do both routes from the beginning, or don't do it at all. Look what happened to Foot of the Mountains. Dev added ntr way into the game, and people were understandably PISSED. You can't cultivate a harem audience then pull the old switcheroo and expect it to go over well.
As for your final point it would not be a success just for the simple fact that the audience for ntr is absolutely miniscule compared to harem. If a dev is only looking to make a successful game avoiding ntr is probably a safer bet. Do note that I'm not saying devs shouldn't make ntr games. People like what they like and who am I to tell them otherwise. It's just not in a devs best interest to add ntr in the late game.