and here Momo needs to save Wano as well. he needs to get Luffy up there and needs to be a huge dragon to save the flower capital it is worth that as well. Momo is the leader of Wano why would Luffy go against what Momo thinks is right?
He doesn't need to be an adult to fly Luffy somewhere...
Luffy was asked to put Wano on his back and fight Kaido for the citizens, that is what is going to happen.
If Oda seriously removes 20 years of a childs' childhood to maybe do one insignificant attack on Kaido then it will leave a bitter taste of Wano, one that ultimately seems unnecessary.
Luffy has been guiding Momo's path this whole way correcting mistakes he makes,
this is just to show Momo's heroic determination and reiterate his frustration that he can't fight Kaido.