it's true that Luffy isn't written by an author…
It's Oda who shows the need for a lookout, as recently as the beginning of egghead, when the crew was ambushed because no one was watching out for danger. It shows a need.
	
	
		
		
	
	
Oda established her from the start as a lookout.
	
	
		
		
	
	
and then on the ship.( WCI squad vs yonko crew btw)