There's no doubt that Hayward would be most comfortable in a 2nd or 3rd option scoring/creating role. He is so unselfish. He's perfect for that role.
Kemba Walker is best suited for a 2nd or 3rd scoring/creating option too, primarily due to his size. He struggles to score against elite length, but if he can attack that length off of creation by someone else, he efficiency goes up.
Jaylen Brown is just not quite good enough yet to carry an offense. I really like his game. More and more reps will get him used to the speed of the game and help him make good reads. Brown is much more suited to be an explosive scorer next to a primary scorer, but not have the offensive consistently revolve around him.
These guys compliment each other well. People that say, "Ainge is lucky that Hayward" blah blah blah don't understand that Ainge has built this team around unselfishness and intelligence. These are the guys he has picked for a reason.