I liked Mittlestadt, but he was always in comfortable circumstances until he turned pro, and I don't think he was used to dealing with adversity. Stayed at his HS rather than Ann Arbor, went to his local college which is all of ten miles away, for the team he grew up watching. Basically, he had it really easy.
Johnson is a different case. He has changed countries, and moved thousands of miles away to play hockey. Was not gifted a spot on a junior international team. Had to deal with Covid affecting his ability to play, or even go home and visit his family. I would bet that Johnson is a little tougher mentally than Casey.