To be fair, a lot of these guys had circumstances around them that I'm not sure any other pitching coach could have helped.
Kopech - First full season as a starter in the big. Looks like he ran out of steam. Solid otherwise.
Keuchel - Released by two other teams. He was just finished.
Lynn - bad knee and came in to season hurt. Had to work his way into shape in season.
Bummer - injured but has been fairly decent when playing.
Crochet - TJS, was inevitable. Not sure how Katz prevents that.
The rest of the relievers? Probably shouldn't be on the big league roster to begin with unless it's mop up duty or desperation. Definitely not to rely on in any large capacity.