Kaguya was a character that Kishi definetely wanted to include, it wasn't just someone that needed to be included just so Madara could be defeated. It might be partially correct but the thing is that this woman had byakugan a dojutsu whose importance was foreshadowed back in the chunin exams with Kakashi. So for Kishi at least, she was truly important.
Now as for the main thing being over, that might be true, but at the end of the day this is bussiness. Kishi at this point had 2 choices: either to tell everything about her in the manga or leave for the movie. If he had done the former, he would also risk because many people were already feeling that this war had dragged for too long, imagine if we were to witness the story of a completely new character. However, the second choice gives him more time to plan the plot for the movie and it will also be a huge success since it wll be the first naruto ever cannon movie. There are people who don't watch movies just because "they're not cannon" but what if they are? I think that the success of the movie can be increased many times.
In any case, I liked Kaguya up to a point, she was something fresh, she wasn't talknojutsued and there wasn't much dialogue as many were whining in the past. So, I believe that it's good to have this opinion now if the following movie does indeed give us important details about her.