And Serbia have Brankica... When you have full confidence in your OH, and you know that you don't need to kill yourself to win a game, you can play way more comfortable and it reflects in your own game, the game flows naturally... Kim and Natalia are not so trustworthy in this level anymore
Brankica has shortcomings in defense, which makes it essential for the use of Busa. She receives all the ball covering brankica. OH is a position you have to both receive and attack.
I don't think kim+natalia is weaker than brankica+busa.
Also as a individual, I barely find better OH than Kim, Natalia. Only Zhu would be better than them, and other OH seems to be in the similar level or lower than them.