McKennie and Arthur are basically half a player each that together would make Khedira.
McKennie doesn't have nowhere near enough talent to be a midfielder at a top club. He has vision to attack the box and run between the lines. But he lacks all the basic skills a midfielder is supposed to have. That's why he gets played only on the wings or as an attacking midfielder. He cannot play the role of a regular midfielder.
Arthur is the opposite. All the skill needed to control the ball, protect it under pressure, and pass accurately. But none of the athleticism or vision.
These two need roles catered specifically to them to cover their big deficiencies, something that can be done at a smaller club but not at Juve. A Fiorentina, Atalanta, or Verona could surely make a good role for them.
At least Rabiot does have a proper midfielder profile. He is lazy and entitled which means he will never fulfill the promise of his talent. But still this lazy underachieving version of Rabiot is still a functional midfielder. He is more of a passenger than a liability when he has a bad game, as opposed to the other two.