In my opinion, this is one of Kishi's themes where kind-"hearted", "open-minded", fresh and youthful young-lings take the lead, control, and power from decrepit, archaic, narrow-minded, old-fashioned "farts".
I think this is his commentary on the situation in his country where it is hard to reach the heights in Japan while being new and young, and how system is oriented to respect, favor, bend-over and execute elder's decisions, while hindering fresh, new ideas. It might even be Kishi's personal hate and commentary that maybe prevented him from achieving some other career goal other than Naruto. Well that is speculation and only my opinion.
In that case, I see why Madara plays such a huge role and becomes very important. He is an old-age tyrant that manipulates others to execute his conclusions about the world, leaving all the youth and new generations without any choice.
I could see Kaguya taking the mantle of such role, but it is stupid to make her FV at this point and simply impossible to make her as good. She has zero character development and manga is coming to an end. Though she can work out as FV partially, if they make Kaguya-Madara merge, and they fight as one hyper-duper-super powered being.