Momo is the symbol of Kozuki and the one who motivated them to stand up and fight.
In their eyes, he is the one inheriting Oden´s will.
Momo is expressing his own doubts because he does not see himself ready, but he is still pushing himself being worthy of that burden, as he states himself, do not cower before it.
Yes, they would have double the problem, first this is feudal Japan and she is a girl.
Whereas there are countless examples of young children taking up the Shogun position in history, with Ietsuna being one of the most famous ones.
You are saying that Oda is gonna forsake all this because of things you see as a problem but was never treated so in the story.